That formula is neither commonly accepted nor correct. Much like the accuracy contribution from combat skill, evasion above 200 skill gives .9 evasion per point.
Just a shame about the enemy critical hate rate -2%. Yeah sure it'll help when shadows are down and stuff but a warrior, paladin or even a samurai could get more use outta this stat.
Decent defence too.
But, nice enough mantle i guess, but there's better, unless you are crazy about the stat.