With the help of some fellow watchers, now I know how the game calculates lash damage.
Suppose "PhyDMG_mid_stage" is the calculated physical damage only without penetration factor. For example, with +60% might, +60% legendary, +60% sneak attack, -50% Graze, the PhyDMG_mid_stage would be Roll*(1+0.6+0.6+0.6-1)=1.8*Roll.
(-50%: 1-1/0.5 = -1, you can check this
<a href="https://forums.obsidian.net/topic/98679-mechanics-attack-speed-recovery-time-reload-time/">mechanics-attack-speed-recovery-time-reload-time</a>
for explanation)
The baseLashDMG = PhyDMG_mid_stage * coef
The