About airfields.
(1) Place stationary planes at spawnpints you don't want (like inside the small hangars).
(2) If all spawn points are taken at an airfield, then planes will spawn around the perimiter of the airfield. The perimiter is the airfield circle that appears in the FMB. If they spawn to far from the airfield then reduce the airfield perimiter circle.
(3) Custom airfields - use ctrl-B to create a branch in the taxi-waypoint pathway.
(a) I've advocated that devs recode airfield bases so if you place a custom airfield object over a hard-coded airfield, then all the hard-coded airfield waypoints are deleted. This would allow you to customise the hard-coded base taxiways & spawnpoints. Sadly, the devs aren't listening
(b) You can't just add to the hard coded airfield taxiway/spawnpoints either. Placing additional taxiways/spawnpoints at a hard coded base does not work either