Potency up

So with tenacity up/down the stats if tenacity gets +/- 99900 tenacity. Being that potency is worded similarly to tenacity down, then potency up doesn't have a number we know of, and (my guess is) potency gets an increased of +99900 potency but that wouldn't be right because that would be it could directly counter tenacity up, and that doesn't happen. I heard it's +25% potency but unless a dev confirms that then we don't know. Now for the entire reason I ask, how does potency up effect Tarkin and his stacking potency when he finally uses his ultimate firepower?