I messed up that calculation. I forgot to tack on the random bit of pDIF and the level correction.
This only applies to TP damage.
If you have a lot more STR and attack than the monster has VIT and DEF, nefarious collar will increase melee damage by 5.16%. If you have such low STR that your fSTR is negative, and if your attack compared to the monsters VIT/DEF, taking into account level correction, is so low that your pDIF->0 nefarious collar will increase melee damage by either 7.69% or ~16 % depending on whether melee damage is rounded up above .5 or just floored... I'm not sure which is true.
Those are the very extreme upper and lower values.
That's what I got, anyway.
It's probably important to note that I've assumed you have capped dDEX and full critical hit rate merits. In the opposite extreme, where your DEX is absolute garbage and dDEX is at it's minimum value, nefarious collar adds at most 30% and at least .24% (low,high pDIF values).
Looking back, the highest values for percentage increase are the case where pDIF=0. This is the case where you wouldn't actually deal damage unless the hit was a critical. I'm not sure this is the best way to analyze this.
Also, these values are very dependent on where the rounding occurs in the formulas, which is something I'm not very sure about. I think pDIF rounds after three decimal points and damage either floors or rounds up at the very last step.