Rancor Tenacity

I believe it drops down to 15%

Seems to drop down to 15%. My Ewok scout with only 28% potency has no problem landing TMR. Don't think it would be possible if the Pit Fighter bonus was still there.

Cool thanks. Just 7 starred dengar and want to use him

Chance to resist = your potency - rancor combined tenacity - 30% tenacity down debuff

It can't drop below 15% resist which is the hard cap (or go above 85% chance to land if you want to look at it that way.