The problem with basing it on numbers in flight is that it potentially leads to big missions having access to lower ENY aircraft than their side's numbers in flight should allow.
B: 160 total, 110 in flight
K: 100 total, 90 in flight
R: 130 total, 115 in flight
Then one second later a 45 player Bishop P-51 mission launches and:
B: 160 total, 155 in flight
K: 100 total, 90 in flight
R: 130 total, 115 in flight
Now no other Bishop can launch a P-51D, but those 45 guys all got P-51Ds to go after the Knights.