You can all stop testing the effects of TH, I've worked it out!
It's the same as Rogue's Roll XD

Die RollTH level .......No THF /THF ......With THFTHF Main
1.....................................2%.................8%
2.....................................2%.................8%
3.....................................3%.................9%
4.....................................4%.................10%
5 (Lucky)..........................12%...............18%
6.....................................5%.................11%
7.....................................6%.................12%
8.....................................6%.................12%
9 (Unlucky).......................1%.................7%
10...................................8%.................14%
11...................................19%................25%
12+ (Bust) .......................-6%................-6%

I'm not quite sure how you get more than TH5 with THF Sub, but nevermind. Maybe BLU/THF will be able to achieve this after their update