Armor Penetration Model?

[Mustang]

I don't know, but it always seemed to me that one inch of steel armor = one armor unit, taking into account armor slant. For example, a steel plate inclined at about 45 degrees is roughly twice as effective against shot than one at 90 degrees, so 1 inch of armor at 45 degrees = just as good as 2 inches of armor at 90 degrees = 2 armor units. It's just what I think, but it seems like an easy way to convert it if noone else offers a better idea.