You said you're going to fix him, but he works the same as yi or any other champ.
Example: This is how Master Yi's E passive stacks with a full stacked guinsoo.
The 10% will give 25/244\*base + 31/244\*flat bonus
Guinsoo will give 6/244\*base + 66/244\*flat bonus
For a total of 31/244\*base + 97/244\*flat bonus
Full Math of what was done above:
Passivebonus = (10% \* (base + flatbonus) + 24% \* 10% \* flatbonus) / (1 - 24% \* 10%)
Guinsoobonus = 24% \* (passivebonus + flatbonus)
By checking this post which says that the stacking is bugged https://www.reddit.com/r/summonerschool/comments/87s9u6/rageblade_on_jhin_is_bugged_and_heres_the_proof/
You have 44% + 100% \* 0.4 + (90+48) \* 0.25 = 118.5%ad on passive
By replacing stuff in the equation above you get:
Passivebonus = 1.655953046 \* base + 7347/3578 \* flatbonus
Guinsoobonus = 0.24 \* (passivebonus + flatbonus)
Since in the example above Jhin had 165 flatbonus ad and 129 base ad he gets a total of 724.608161 bonus ad which summed with his flatbonus and base get 1018.61 total ad which is equal to the "bugged" result the guy had in his post.
So... are you going to make Jhin an exception because he synchronizes with this so well? Or are you going to do it for all champs? Master Yi, Rengar, Olaf, RIven, Nunu, Vladimir?
I posted the formula so you can check it yourself.
PS: Jhin also stacks additively with dragon, unlike other champions which stack recursively. I think they'll make guinsoo stack like it. At least they're making it consistent. But still sucks that other champs get more ad than him from dragon before his late game passive surpasses them.