[Edi]

That armor calculation looks correct. I had it somewhere sometime, but either I have lost the email, or just can't find it, or it was in some other venue.

In any case, the armors have hidden attributes for leg and arm protection, which were later not implemented, but which enter into the calculations, which results in all those mysterious additions and multipliers, but which end up with the body prot values we know. The interactions between natural protection, earth magic and so on are then factored in and it's adjusted, so that full earth magic and natural prot are not added.

That's because if they were, high natural protection units would become invulnerable to anything other than armor negating weapons or something that did times x multiplied damage against them (with high strength).