Transmute: Shadowspirit Diamond

There is no cooldown on this, I just made 10 of them.

My current data:

Transmutes cast - 431
Total diamonds received - 1007
431/1007 = 233.64% (200% guaranteed)
33.64% extra, or 33.64/2 = 16.82% Procrate.

This seems higher than the 15%. Although my numbers could on the high side, it seems like 15% isn't what is going on here.

Other comments (which I oddly can't reply inline to anymore) suggest proc rates of 10-15%

As the elixir-specced flask crafter for our guild who is sometimes crafting 100+ flasks at a time (thus I have large sample sizes), I've been using a 1/6 proc rate estimate (16.666667%) for quite some time in factoring costs and it seems to concur with what I get. I assume the proc rates for all proccable items -- at least alchemy ones -- are roughly the same. (And if I had to guess how many times other things proc -- JC items being blue rather green for instance -- my gut instinct says around 15% as well.)

The one possible damper in this is the fact that the "first" proc of a shadowspirit diamond doesn't double your yield but instead adds one extra -- but it's possible you simply get twice as many chances to proc (i.e. each of the two shadowspirit diamonds gets a separate opportunity to proc).
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The 1/6 proc rate estimate accounts for multiple procs -- so it's possible that the 'true' proc rate is 15%, and each created item (whether it be from the base recipe or a proc or a proc of a proc...) has a 15% chance to proc an additional -- this would 'pad out' the 15% to be 'about' 1 in 6.