I use a text editor to setup spawns files.
Exemples:
1) this one will spawn the aircraft, object, ship or whetever with 100% chances at every altitude
LowAgl="3.0479998588562" InRangeProbability="100" OutOfRangeProbability="100"
2) this one will spawn the aircraft, object, ship or whetever with only 20% chances, whatever the player altitude is.
LowAgl="3.0479998588562" InRangeProbability="20" OutOfRangeProbability="20"