OK, I've had characters with as high as 27 evasion, never saw anything actually miss them.
even tried blinding a few enemies... doesn't seem to affect much.
either it's bugged, or it isn't anywhere close to a straight percentage, or else monsters all have accuracy boosts that overcome most evasion anyway (assuming evasion/accuracy counter each other like damage/protection).
anyone try a pure evade type character? does it work, or not?
I ended up going for protection instead of evasion, as it seemed more reliable.