BLM

[jcw005]

I highly doubt your mathematics here, sorry.

Can't imagine that a 5% per tick chance makes it to a ~ 30% procc chance in the end.

You misunderstand. What Rbstr is saying is that with a 5% probability of firing at each tick and 8 ticks per Thunder 3, over the course of the entire Thunder 3, there is a 34% chance that Thunderstorm will have triggered at some point during that duration. In other words, if you were to use cast Thunder 3 100 times, and let it tick its full duration, roughly 34 of those would proc a Thunderstorm at some point.
The probability of an event occurring at least once over a series of tries is obviously greater than the probability it occurs on a single event. Flip a coin a hundred times and the probability of at least one head is much, much greater than 50%.