I built a classic creeper farm with cats in Java version 1.16.5. It is in a Plains biome and is not in the air.
Lower end: height 71
Upper end: height 94
At what height in the air above the farm should I build an AFK platform to allow a maximum number of creepers to spawn?
At what level is the point that the creepers die? 71? Or is that the level from the lowest spawn room?
Add 127 to the lowest level where the creepers ever are (probably where they die). Your AFK spot must be there.
Now found 100 blocks above the farm to be the most effective point. (the creepers die on 72)
So 122 blocks over the point where they die? Yeah, that fits perfectly