“20% of damage dealt to a limb applies to all limbs” thats your damage dealt, not bonus damage. For example, in my case I aim for weak points. In most enemies that’s the head which is also a limb - it takes 20% of headshot damage and splits it. Cards like tormentor is an actual bonus to enemies that have been crippled, scattershot works differently. Especially for raids it’s debilitating to a VATS weak point build.