Someone can give a more thorough explaination later but heres what my understanding of it.
It's only the 5" CV guns that have proximity fuzes in AH.
Historically the proximity fuze was used by the US Navy from about 1943, and functioned by a sort of radar.
The Germans didn't use proximity fuzes. Their large FLAK batteries were set to go off at a certain altitude. This was set by adjusting the flight time (or how many times the round had spun). It's straight forward to calculate from flight time to altitude.
