why are you multiplying by 4 instead of 2?
geostationary orbit is 35,786km above sea level.
so round trip would be 71,572km.
speed of light is exactly 300,000km/second.
(it's exact because the definition of a meter is based on speed of light)
300,000 / 71,572 = 0.23857333 seconds
100ms equipment overhead? are you using a 1200b/s serial cable to connect to your satellite modem? i can ping google at 14ms. surely the backbone connection at the base station can beat my connection. 239+14=253ms theoretical speed. i expect no more than 300ms even during the busy time of the day.