So far I've done about 60 Snake Eye attempts, excluding rolls that were at 10 beforehand for obvious reasons, and out of 60 attempts, 31 were successful at giving an instant 11, so for now I'm inclined to believe that the in-game text is correct about 10% per merit. Of course, this could be a mistake on part of the person who programmed the merit, and this may see an adjustment soon.
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Give players their freedom... they'll bind it in chains.