Okay, let's see. I'll link my work and then try to explain a bit.
Source
us.battle.net/wow/en/forum/topic/8197741003
Trinket functionality change
1st page
oi46.tinypic.com/ei8ub6.jpg
Contains the previous results at start. After that I created the function for the multiplier.
Also, we can easily see the multiplier effect starts at 1.5*expected time between procs, based on the blue post.
After that I just changed the new "chance to not proc in T time from last proc" to the form with e.
2nd page
oi45.tinypic.com/k9gbcp.jpg
Basic change to proc in T for both T>=1.5*E(T) and T<1.5*E(T) , in other words cumulative probability distribution.
Graph to make it easier to understand:
oi47.tinypic.com/28ic1mh.jpg
H=R=1.0
As we can see, at 1.5*E(T) the lines cross, and after that the multiplier kicks in, resulting in the probability approaching 1 much faster than before the change.
After that I have the derivatives for both functions, as well as the real expected value Er(T) for T<1.5*E(T).
3rd page
oi46.tinypic.com/2cqcsax.jpg
This is where the stuff got a bit more nasty as I said. What I did was create an approximation using T= 90/(HR) = 1.5*E(T) for the T before the integral, resulting in a much easier approach.
Using this I got a very easily useable approximation for the total expected value Et(T).
4th page
oi50.tinypic.com/258qmub.jpg
After this I did the real expected value for T>1.5*E(T), which led to error function (erf).
Using this I could make a couple comparisons between the real result (real) and my approximation version (short).
5th page
oi50.tinypic.com/xaxdo6.jpg
Here are the real values of E(T) when T>1.5*E(T) in a nice and tidy format.
After getting this I just summed the E(T) ,T<1.5*E(T) and E(T) ,T>1.5*E(T) to get the real expected total time between proc.
To my surprise this led to a very similar formula to my approximation, the difference in the expression being the error of approximation function.
Now, this does not take into account ICD, which I haven't gotten into yet. However, comparing this to the newest simulationcraft results they seem to be close, the difference being made by the guaranteed proc at the start of the fight as well as the possible effects of ICD.