Talk:Magic penetration/@comment-69.14.62.169-20111215025503/@comment-2066615-20111218193224

Percentage M.pen does not stack additively, only flat M.pen stacks additively. Percentage M.pen are stacked multiplicatively, resulting in 46% total M.pen, not 50%.

The same logic which is used to acquire highest possible % M.pen Xerath can obtain, which is 30 + 40 + 10 in %. Final result is not 80%, but instead calculation are done in this way:

First 30%, so flat 30% as usual. Assuming enemy has 100 M.res, this reduce enemy M.res to 70.

Then next 40%, after first M.pen, reduce from that result, which is 70. Resultant M.res after this is 42.

Lastly, Mastery's M.pen of 10% is added, which reduce that 42 M.res to 37.8 M.res.

Resulting total reduction is 100 - 37.8 = 63.2% Total reduction.