If I have 25% bonus crit damage from runes and another 50% from IE, how much will Jhin's crits do?

250%

I recommend going all the way, you can get something like 276%, which is enough to one-shot some champs come late-game

It equals "tons of damage". Ur welcome Phreak.

League wiki says: Jhin's critical strikes deal 25% reduced damage

So if you have IE (50%) + 25% crit damage from runes then a crit would normally deal 275% AD With Jhin it would deal 275% X 0.75 = 206.25% AD

150 X 1,25 (IE) X 1,125 (Runes) = 210,9375

He would deal 211 % of his AD per crit.