According to that thread mastery chance got reworked at some point, and 300 nets you just above 60% on PC. No idea how it is on console.
It is often recited that Jinx affects masteries themselves not the % directly, though with my experience of a single Jinx dipping my surge % as low as in the 30-40% range over several battles(again small samplesize: inconclusive) i am highly suspicious there is more to Jinx than that, though it might just be bad luck.
In any case registering surge chances against a single Jinx proves to be impractical these days cause you know you only meet teams with two of them^^
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