Really depending on context and talking to Hitech, you can space small airfields 13miles apart. As for the 18.75, it's easier to just do 19 miles with 18.75 allowing for situational fudging. Precedence helped on Oceania with the central island and airfields 13 miles apart due to NDisles. I would confer with Hitech on anything more than a map center setup like NDisles and Oceania. You would run into many player behavioral problems with a map full of 13 mile separations for airfields.
