Disregard AirlineFlightSchedules. That is not necessary for you to retrieve near-term future flights. I mentioned it only if you needed far future flights.
FlightInfoEx already returns a small number of future flights when you call it. For the exact flight you just mentioned, one of the first two rows in the returned result represents the future flight you wanted. See the part I have highlighted in bold.
{“FlightInfoExResult”:{“next_offset”:15,“flights”:{“faFlightID”:“UAL450-1365743875-airline-0143”,“ident”:“UAL450”,“aircrafttype”:“B752”,“filed_ete”:“04:42:00”,“filed_time”:1365744748,“filed_departuretime”:1365981600,“filed_airspeed_kts”:379,“filed_airspeed_mach”:“”,“filed_altitude”:0,“route”:“”,“actualdeparturetime”:0,“estimatedarrivaltime”:1365999120,“actualarrivaltime”:0,“diverted”:“”,“origin”:“KCLE”,“destination”:“KLAX”,“originName”:“Cleveland-Hopkins Intl”,“originCity”:“Cleveland, OH”,“destinationName”:“Los Angeles Intl”,“destinationCity”:“Los Angeles, CA”},{“faFlightID”:“UAL450-1365658357-airline-0017”,“ident”:“UAL450”,“aircrafttype”:“A320”,“filed_ete”:“05:54:00”,“filed_time”:1365658357,“filed_departuretime”:1365865200,“filed_airspeed_kts”:361,“filed_airspeed_mach”:“”,“filed_altitude”:0,“route”:“”,“actualdeparturetime”:0,“estimatedarrivaltime”:1365887040,“actualarrivaltime”:0,“diverted”:“”,“origin”:“KEWR”,“destination”:“KLAX”,“originName”:“Newark Liberty Intl”,“originCity”:“Newark, NJ”,“destinationName”:“Los Angeles Intl”,“destinationCity”:“Los Angeles, CA”},{“faFlightID”:“UAL450-1365485620-airline-0023”,“ident”:“UAL450”,“aircrafttype”:“A319”,“filed_ete”:“00:36:00”,“filed_time”:1365742639,“filed_departuretime”:1365740940,“filed_airspeed_kts”:480,“filed_airspeed_mach”:“”,“filed_altitude”:220,“route”:“IDU4 IDU WLEEE BITER6”,“actualdeparturetime”:1365743280,“estimatedarrivaltime”:1365744929,“actualarrivaltime”:1365745080,“diverted”:“”,“origin”:“KIAH”,“destination”:“KAUS”,“originName”:“Houston Bush Int’ctl”,“originCity”:“Houston, TX”,“destinationName”:“Austin-Bergstrom Intl”,“destinationCity”:“Austin, TX”},{“faFlightID”:“UAL450-1365399154-airline-0017”,“ident”:“UAL450”,“aircrafttype”:“A320”,“filed_ete”:“00:28:00”,“filed_time”:1365655054,“filed_departuretime”:1365654360,“filed_airspeed_kts”:387,“filed_airspeed_mach”:“”,“filed_altitude”:200,“route”:“IDU4 IDU WLEEE BITER6”,“actualdeparturetime”:1365654240,“estimatedarrivaltime”:1365656640,“actualarrivaltime”:1365656700,“diverted”:“”,“origin”:“KIAH”,“destination”:“KAUS”,“originName”:“Houston Bush Int’ctl”,“originCity”:“Houston, TX”,“destinationName”:“Austin-Bergstrom Intl”,“destinationCity”:“Austin, TX”},{“faFlightID”:“UAL450-1365312608-airline-0037”,“ident”:“UAL450”,“aircrafttype”:“A319”,“filed_ete”:“00:44:00”,“filed_time”:1365568342,“filed_departuretime”:1365567960,“filed_airspeed_kts”:387,“filed_airspeed_mach”:“”,“filed_altitude”:200,“route”:“IDU4 IDU WLEEE BITER6”,“actualdeparturetime”:1365568320,“estimatedarrivaltime”:1365569925,“actualarrivaltime”:1365570060,“diverted”:“”,“origin”:“KIAH”,“destination”:“KAUS”,“originName”:“Houston Bush Int’ctl”,“originCity”:“Houston, TX”,“destinationName”:“Austin-Bergstrom Intl”,“destinationCity”:“Austin, TX”},{“faFlightID”:“UAL450-1365139668-airline-0058”,“ident”:“UAL450”,“aircrafttype”:“A320”,“filed_ete”:“04:00:00”,“filed_time”:1365374403,“filed_departuretime”:1365373740,“filed_airspeed_kts”:456,“filed_airspeed_mach”:“”,“filed_altitude”:360,“route”:“COATE J36 FNT MKG BAE J16 MCW J148 ONL KIPPR2”,“actualdeparturetime”:1365376560,“estimatedarrivaltime”:1365390240,“actualarrivaltime”:1365390480,“diverted”:“”,“origin”:“KEWR”,“destination”:“KDEN”,“originName”:“Newark Liberty Intl”,“originCity”:“Newark, NJ”,“destinationName”:“Denver Intl”,“destinationCity”:“Denver, CO”},{“faFlightID”:“UAL450-1365052652-airline-0156”,“ident”:“UAL450”,“aircrafttype”:“A319”,“filed_ete”:“00:52:00”,“filed_time”:1365272947,“filed_departuretime”:1365270660,“filed_airspeed_kts”:424,“filed_airspeed_mach”:“”,“filed_altitude”:260,“route”:“WAILN1 WAILN RODIO1”,“actualdeparturetime”:1365273480,“estimatedarrivaltime”:1365275640,“actualarrivaltime”:1365275520,“diverted”:“”,“origin”:“KIAH”,“destination”:“KSAT”,“originName”:“Houston Bush Int’ctl”,“originCity”:“Houston, TX”,“destinationName”:“San Antonio Intl”,“destinationCity”:“San Antonio, TX”},{“faFlightID”:“UAL450-1365052652-airline-0155”,“ident”:“UAL450”,“aircrafttype”:“A319”,“filed_ete”:“02:30:00”,“filed_time”:1365258451,“filed_departuretime”:1365257820,“filed_airspeed_kts”:442,“filed_airspeed_mach”:“”,“filed_altitude”:380,“route”:“ACITO ADELL ARLYN STL J101 LIT J180 SWB TXMEX1”,“actualdeparturetime”:1365259260,“estimatedarrivaltime”:1365267918,“actualarrivaltime”:1365267900,“diverted”:“”,“origin”:“KORD”,“destination”:“KIAH”,“originName”:“Chicago O’Hare Intl”,“originCity”:“Chicago, IL”,“destinationName”:“Houston Bush Int’ctl”,“destinationCity”:“Houston, TX”},{“faFlightID”:“UAL450-1365052652-airline-0154”,“ident”:“UAL450”,“aircrafttype”:“A320”,“filed_ete”:“02:32:00”,“filed_time”:1365245778,“filed_departuretime”:1365246000,“filed_airspeed_kts”:441,“filed_airspeed_mach”:“”,“filed_altitude”:380,“route”:“COATE J36 FNT WYNDE4”,“actualdeparturetime”:1365246300,“estimatedarrivaltime”:1365253380,“actualarrivaltime”:1365253380,“diverted”:“”,“origin”:“KLGA”,“destination”:“KORD”,“originName”:“LaGuardia”,“originCity”:“New York, NY”,“destinationName”:“Chicago O’Hare Intl”,“destinationCity”:“Chicago, IL”},{“faFlightID”:“UAL450-1364966264-airline-0095”,“ident”:“UAL450”,“aircrafttype”:“A319”,“filed_ete”:“02:21:00”,“filed_time”:1365176061,“filed_departuretime”:1365175260,“filed_airspeed_kts”:447,“filed_airspeed_mach”:“”,“filed_altitude”:370,“route”:“SFO9 SFO LIN PEONS INSLO DTA J84 EKR KAILE1”,“actualdeparturetime”:1365176220,“estimatedarrivaltime”:1365182880,“actualarrivaltime”:1365183060,“diverted”:“”,“origin”:“KSFO”,“destination”:“KDEN”,“originName”:“San Francisco Intl”,“originCity”:“San Francisco, CA”,“destinationName”:“Denver Intl”,“destinationCity”:“Denver, CO”},{“faFlightID”:“UAL450-1364879874-airline-0193”,“ident”:“UAL450”,“aircrafttype”:“A319”,“filed_ete”:“01:49:00”,“filed_time”:1365111373,“filed_departuretime”:1365111600,“filed_airspeed_kts”:441,“filed_airspeed_mach”:“”,“filed_altitude”:390,“route”:“YOKES2 DEHLI RWF SKETR5”,“actualdeparturetime”:1365111960,“estimatedarrivaltime”:1365117311,“actualarrivaltime”:1365117600,“diverted”:“”,“origin”:“KDEN”,“destination”:“KMSP”,“originName”:“Denver Intl”,“originCity”:“Denver, CO”,“destinationName”:“Minneapolis/St Paul Intl”,“destinationCity”:“Minneapolis, MN”},{“faFlightID”:“UAL450-1364879874-airline-0192”,“ident”:“UAL450”,“aircrafttype”:“B752”,“filed_ete”:“02:45:00”,“filed_time”:1365094599,“filed_departuretime”:1365088860,“filed_airspeed_kts”:460,“filed_airspeed_mach”:“”,“filed_altitude”:350,“route”:“SFO9 SFO LIN PEONS INSLO DTA CHE FRNCH1”,“actualdeparturetime”:1365096120,“estimatedarrivaltime”:1365103111,“actualarrivaltime”:1365103140,“diverted”:“”,“origin”:“KSFO”,“destination”:“KDEN”,“originName”:“San Francisco Intl”,“originCity”:“San Francisco, CA”,“destinationName”:“Denver Intl”,“destinationCity”:“Denver, CO”},{“faFlightID”:“UAL450-1364794122-airline-0029”,“ident”:“UAL450”,“aircrafttype”:“A320”,“filed_ete”:“04:19:00”,“filed_time”:1365029541,“filed_departuretime”:1365028140,“filed_airspeed_kts”:436,“filed_airspeed_mach”:“”,“filed_altitude”:280,“route”:“COATE J36 FNT J547 OBK DSM OVR SPAWN KOHOE1”,“actualdeparturetime”:1365030120,“estimatedarrivaltime”:1365042900,“actualarrivaltime”:1365043080,“diverted”:“”,“origin”:“KEWR”,“destination”:“KDEN”,“originName”:“Newark Liberty Intl”,“originCity”:“Newark, NJ”,“destinationName”:“Denver Intl”,“destinationCity”:“Denver, CO”},{“faFlightID”:“UAL450-1364707684-airline-0023”,“ident”:“UAL450”,“aircrafttype”:“A319”,“filed_ete”:“01:25:00”,“filed_time”:1364947390,“filed_departuretime”:1364939640,“filed_airspeed_kts”:455,“filed_airspeed_mach”:“”,“filed_altitude”:390,“route”:“DUFEE ELX HAAKK DOXXY SOSIC KEEHO J584 SLT FQM3”,“actualdeparturetime”:1364948280,“estimatedarrivaltime”:1364954280,“actualarrivaltime”:1364954460,“diverted”:“”,“origin”:“KORD”,“destination”:“KEWR”,“originName”:“Chicago O’Hare Intl”,“originCity”:“Chicago, IL”,“destinationName”:“Newark Liberty Intl”,“destinationCity”:“Newark, NJ”},{“faFlightID”:“UAL450-1364707684-airline-0022”,“ident”:“UAL450”,“aircrafttype”:“A320”,“filed_ete”:“03:29:00”,“filed_time”:1364921151,“filed_departuretime”:1364921220,“filed_airspeed_kts”:448,“filed_airspeed_mach”:“”,“filed_altitude”:350,“route”:“TNP PKE DRK FLG J10 RSK J64 PUB HGO HCT FOD J94 DBQ JVL BULLZ3”,“actualdeparturetime”:1364921640,“estimatedarrivaltime”:1364932861,“actualarrivaltime”:1364933040,“diverted”:“”,“origin”:“KPSP”,“destination”:“KORD”,“originName”:“Palm Springs Intl”,“originCity”:“Palm Springs, CA”,“destinationName”:“Chicago O’Hare Intl”,“destinationCity”:“Chicago, IL”}]}}