This is a suggestion which has been made many times over the years, maybe some day Bytro will prioritize all of the 'please make game easier' suggestions, in mean time...
You accomplish this math with travel time of any unit and you move something out to next approximate airfield location to see if it is within the 250km range of a fighter plane.
example > inf unit moves at 36km per hour so you want airfields within 6.75 hours travel time when measuring distance with inf unit on your own terrain, calculating travel time is more complicated if you have ally or enemy provinces in the middle, so in these cases you take into consideration movement times on different terrains.
With some experience you can almost 'eyeball' or guestimate the 250km distance, when it is questionable that is when you do measurement of distance using unit movement times... Good luck and have Fun
