Which set of armour values is correct? None and all
Here I may come with my part (based on some guyesstimates and so on, tried to take into account armour hardness etc):
Hull:
Front 38cm KE/42cm CE
Side 11/13, rear 13/14
Turret:
Front 27/30
Side 18/20, rear 12/13
For rounded surfaces I usually use "approximation" of 33 degree slope (not too scientific I admit
). More or less if I feel the exact shape mandates it.
As for the formula, yep, effective value = thickness/cosinus