"Pretty big armor plate"..? The only plane I've ever seen with a large armor plate behind the pilot is the P-47D. That was 3/4 inch armor plate steel. 109 E and F models had a 1/2 inch plate behind the pilot's head.
If you look at enough pictures, and read enough about a plane, eventually you'll come across armor thickness. Then again, I got lucky; my grand-dad flew 109 F thrugh 109 K models. He might not know the exact thickness, but he can telly you what it sounded like when a round hit that plate!
Flakbait