I did my own testing. Pretty easy : go gather a pack to 8-10 low levels ennemy somewhere, without shield, let them hit you for 5-10 minutes, do the same with only your weapon so you are stripped of almost all your parry. First case if bis for parry you'll get about 28% parry (i have about 26% missing TT head), second case you'll drop to 12 or so if your weapon have parry, or 10 if it doesnt. I did this a numlber of time, used bosses, whatever. Everything is consistent with 10% parry at 321 or whatever the base stat number is at 50, then 1% per 13 parry. That is also what was tested very early in the game, so that's why i was surprised with number in this new thread.

Don't forget never to use a shield btw. Block is computed before parry, so if you have 40% block rate you will reduce the parry rate you observe by 40%. Also don't forget to dismiss all the misses from low level ennemies.