from what I can see when running sims, I test with germany attacking with every landunit that can reach (7 I , 4 M, 3 A, 6 T ) V (1 AAA, 6 I, 2 A, , 2T , 1 Ftr) it gives germany 99.9% chance of winning about 24 IPC of loss

If you compare that too

(7 I , 4 M, 3 A, 6 T, 2 F, 2Tac ) V (1 AAA, 12 I, 2 A, , 2T , 1 Ftr) , it gives about 99% chance of winning with 46 IPC loss

So, those 18 IPC used, gives immidiatly about 22 IPC lost for germany, and keeps 2 ftrs and 2 tacs away from the brit.

(if germany brings less planes to france, the chance of winning for the defender goes up fast, and the losses goes upp really fast)