Hello,
From your FAQ, you suggest the calculation of arrival delay of a flight using the following formula:
How late the arrival was = (actualarrivaltime - (filed_departuretime + filed_ete)).
I’m using that formula on FlightInfoStatus, the problem is that the results seems inaccurate!
Do you suggest to use the arrival_delay value instead to obtain the desired result?
Consulting that value, seems for correct than the computed one.
Thanks
AS example, please use 
https://flightxml.flightaware.com/json/FlightXML3/FlightInfoStatus?ident=CPA779
and do the math with the first flight CPA779-1557900600-schedule-0004:
1558096495 - (1558073400 + 15540 ) = 7555 seconds
“arrival_delay”: 6060, → seems more accurate!
The FAQ method is a recommendation when using endpoints like FlightInfoEx. It’s not needed for endpoints like AirportBoards and FlightInfoStatus that use the FlightInfoStatusStruct and provide a dedicated field for the value.