Boss and total damage stacks in KMS.

You have those potentials in your Pap vid, but I can't make heads or tails of these numbers.

14506 DoT assuming level 1 Venom gives a range of 19159~27370.

Doing Damage Range Value * Final Cut * Phantom Blow * Boss Damage * PDRate.
19159*1.4*1.05*1.6*(1-(0.25*0.6)) and 27370*1.4*1.05*1.6*(1-(0.25*0.6)) gives 38k~54.7k, but you're doing way more in the video.
But I probably did the math wrong, I have no idea how to do factor in PDRate and Ignore PDRate skills.

Oh.
Supposing 14506 DoT, range would be 19159~27370.
Damage formula: [((Damage Range*Skill%)*Critical)*((1 - PDrate) + (PDrate * (ignore defense total))]* DamageInc
Shadow Partner Formula: (Damage/2)*Crit*DamageInc.
As far as I know, Dual Bladers have only 20% ignore PDR from Phantom Blow and have 20% boost to minimum critical rate. But I'll go with your 25% value regardless.
Assuming damage range: 19159~27370.
Critical Damage: 1.4 Minimum, 1.5 Maximum.
Total PDR Ignore: 40%

 Spoiler

Supposing they stack additively.
Maximum Critical: floor[((27370*1.05)*1.5)*((1-.25)+(.25*.40))]*1.6 = 58626
Maximum noncritical: floor[(27370*1.05)*((1-.25)+(.25*.40))]*1.6 = 39084
Minimum critical: floor[((19159*1.05)*1.4)*((1-.25)+(.25*.40))]*1.6 = 38302
Minimum noncritical: floor[(19159*1.05)*((1-.25)+(.25*.40))]*1.6 = 27359
Critical Range: 38302~58626 (SP Range: 42989~70351)
Noncritical Range: 27359~39084 (SP Range: 21887~31267)
I'm supposing Final Cut goes through all this process and is applied lastly.
Final Cut Critical Range: 53622~82076 (SP Range: 60056~98491)
Noncritical Final Cut Range: 38302~54717 (SP Range: 30641~43773)

Supposing Final Cut is additive with Boss Damage:
Maximum Critical: floor[((27370*1.05)*1.5)*((1-.25)+(.25*.40))]*2 = 73283
Maximum noncritical: floor[(27370*1.05)*((1-.25)+(.25*.40))]*2 = 48855
Minimum critical: floor[((19159*1.05)*1.4)*((1-.25)+(.25*.40))]*2 = 47878
Minimum noncritical: floor[(19159*1.05)*((1-.25)+(.25*.40))]*2 = 34198
Critical Range: 47878~73283 (SP Range: 38302~58626)
Normal Range: 34198~48855 (SP Range: 27358~39084)

Too lazy. Maybe I'll finish later. Someone check my work.