[DRG]

What you probably need to do is find a known reliable source that gives 0 and 30-degree pen results for anything similar and then use that as a guide..if it gives 130 pen at 30-degree slope but 135 at zero then you have a "good enough" approximation. ArmourCalc just gives you a good idea of what the horizontal differences is for a piece of armour at a set thickness but there are so many factors interconnected for a projective penetrating armour plate you only ever will be " close enough"

I would say for your example that penetration for that guns shell on a flat 0-degree plate would be slightly more than what it is for 30-degree but it's not going to be wildly different.

I have no opinion one way or the other on stats from other games mainly because I have never looked at them or compared them to "official" sources