Gamparse can give a lot of info on these sort of skills. Here is the # and percentages for a 54 warrior in planar quality gear tanking basement over the course of 50 fights. Not sure if this helps answer your question at all, but you could download gamparse and run some tests yourself with different gear.
Attempts 1371 100%
Missed 590 43.03%
Dodged 44 3.21%
Parried 78 5.69%
Blocked 0 0%
Riposted 49 3.57%
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