I remember back a long time ago I took a drow assassin with perfected parry/dodge/shieldblock and logged a bunch of combat vs. some mobs, then broke out how many I parried, shield blocked and dodged. I forget what order they're checked in, but I took that into account at the time. In the end, I shieldblocked 50% of the melee hits that got that far. The fact that it was almost exactly 50% suggested to me that it was a fixed value. Though, I never did try with a less dexterous version of that build.