The big problem with it’s proc chance is that back in vanilla, 5% really meant 5%. There were no unlucky streak prevention mechanisms. There were times it could proc back to back(if it didn’t have an internal cooldown) and then other times, you’d go over a minute without a single proc.
The equation to solve your probability of getting a successful proc vs number of consecutive attacks is as follows:
1-((1-x)^y)
x=proc chance
y=number of attacks
By attacks, I mean auto attacks+instants+windfury procs+etc
So for example, on your first attack, you will have a 5% chance to proc. If you haven’t gotten a proc by your 5th attack, it will have ~23% chance. For your attack to have a 50% chance to proc, you’d have to attack ~13.5 times in a row. If you were to get extremely unlucky, to have a 95% chance to proc on attack, you’d have to attack 58 times in a row.
And these percentages don’t mean that it literally has a 95% chance to proc, it means that the odds of going that long without a proc are pretty slim. Like flipping a coin 50 times and having it land only on tails, it’s very unlikely.
25% = 5.6 attacks
50% = 13.5 attacks
75% = 27 attacks
95% = 58 attacks
You can input the following into WolframAlpha to see a curve:
1-((1-0.05)^y), plot y=1 to 100
An enhancement shaman example:
Let’s say you auto+stormstrike+WF proc, that’s 4 strikes at once and that means you’ll have ~18.5% chance of procing ironfoe.
Without doing a bunch of math to calculate the exact effect flurry has on your average swing time, I’m just going to say it takes you from 2.4s to 2s swing time on average. That means you get 30 swings per minute+3 stormstrikes=33 attacks, which gives an average of 6.6 windfury procs(WF can’t proc off itself in 1.12). That means that overall, in a minute, you’re averaging 33+(6.6*2)=~46 swings per minute, which gives you a 90% chance of having at least one proc in that one minute.
TLDR: It’s definitely a very powerful weapon and is BiS for some builds, however, it can be pretty unreliable. For most builds, assume at least one proc per minute, but don’t get your hopes too high like it’s going to proc 10 times per minute.
/End rambling math rant