Darn it - someone has correctly pointed out that the bombers in central US could hit z109 but they could not hit z110!
So I recalculated a couple of these and adding 1 carrier with 2 fighters landed on it gets the odds of an Allies win down from 100% to 70.2%; adding 2 carriers with 3 fig and 1 tac gets it down to 7.1%. So it seems that 2 carriers would probably do the job most of the time, but that’s still an awful lot for Germany to spend.
Apologies for my oversight.