Maybe I found Damage formula of Somersalt Kick.

Knuckle Mastery

Source: I am a Bishop

 

Well, he knows something that's for sure.

 

Actually I experimented with this when I was playing a pistolwhipper and I found that the 0.5x multiplier only applies to the Primary stat, and not the rest of the formula.
So it should be
max : ((DEXx3.6x0.5)+STR) ATK/100
min : ((DEXx3.6x0.09x0.5) +STR) ATK/100
it's hard to see if you don't have much STR, but I made 2 characters one with high DEX low STR, other with high STR low DEX, and the results were clear that the STR part wasn't being multiplied by the 0.5.
It's also a significant difference because it means that the DEX multiplier is between 1.8 and 0.162, which averages out to an 0.981 multiplier, less than the 1x multiplier of STR, so the optimal strategy is to put all points in STR for gun somersault kick damage. (although DEX will help more with other pistolwhipper skills like grenade and stuff)

Pretty sure 0.09 is correct, from what I've seen the base mastery stat without any skills is 0.1, and the minimum damage formula has 0.9*Mastery as the multiplier.