Post: #7

There could be a mathematical formula for your case, but you have to prefix a maximum armor achievable and with that maximum the percentage of reduction you want.
Also you have to choose if every point of armor has a flat corresponding % of reduction or some particular behavior, like the next armor point gives less than the preceding.

In the case you want armor to be flat %, simply use a proportion to calculate the reduction:
max_reduction : max_armor = actual_reduction : actual_armor => actual_reduction = max_reduction * actual_armor / max_armor

In the other case tell us what kind "curve" you want and maybe can be solved with a formula