Yes, heywhatsthat provides a very handy tool. One can easily know what he can get.
Due to line-of-sight nature of microwaves, and curvature of earth, the topography around antenna location is an important factor determining maximum range.
Raising your antenna may help. You can re-plot heywhatsthat curves using antenna heights 30 ft and 35 ft (instead of your existing 25 ft) to see how much benefit you will get by raising your antenna
If I recall correctly, RG58 has an attenuation of about 21 dB / 100 ft at 1000 Mhz. For 50 ft length you have, the attenuation in coax is about 10.5 dB. As the Flightaware ProStick has an 18 dB integral RF pre-amplifier, this 10.5 dB attenuation will be easily covered by it.
Another factor which can affect reception is presence of strong Cell phone signals, which can overload/saturate front-end of the receiver (ProStick). Adding a filter may resolve this issue.
Do you have Cell phone towers or building-top antennas in vicinity?
I live in Toronto urban area with lot of Cell phone antennas around. Below is the RF scan at my location.