MK VII 75mm
Accuracy Far: 0.025
Penetration Far: 105
StuG III G
Target size: 17
Front Armour: 140
Chance to hit : 0.025*17=0.425, ~43%. Not accounting for scattered shots.
Chance to pen = 105/140=0.75, 75% chance of a long range pen on an unvetted StuG
So not accounting for scattered shots, a 31.875% chance each shot will penetrate a max range StuG. Worse if it is vet 2.
A StuG has 560 health, which takes 4 penetrating hits to kill. With each shot having a 31.875% chance to hit and pen, that equates to a rough total of 12.5 shots on average to remove a StuG.
StuG 75mm
Accuracy Far: 0.04
Penetration Far: 170
MK VII
Target Size: 26
Front Armour: 240
Chance to hit: 0.04*26=1.04, 100% chance to hit
Chance to pen: 170/240=0.7083..., 71% chance to pen
A MK VII has 1400hp which takes 9 penetrating hits to kill. Every shot will hit and has a 70% chance to penetrate, that equates to roughly 12.9 shots on average to kill a MK VII.
None of the above is accounting for moving accuracy, scattered shots (only applicable to the MK VII or moving StuG), or nay bulletins applied.
You can also double some of the stats for the Stug as there should be at least two of them fighting a churchill at that stage at least.