No, its completely random. There is sometimes where I'll go for what seems like forever without a proc and then get 3 in the matter of 30 seconds (welcome to FFXI). Just eyeball testing after doing 6 yacumama I would say that the generally accepted 6-10% proc rate is pretty accurate. Its still pretty nuts though, tanking that mob pld/war and never having to heal yourself because it keeps healing you back to full HP.
It only seems high because if you think about it, 6-8% proc rate is 1 proc every 15 attacks.
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