From the FAQ: “If your PTH is lower than 70, any attacks that land will do reduced damage. The damage reduction multiplier is equal to your PTH / 70 (ex. if your PTH is 65, you will do 92.86% of normal damage on a hit, or 65/70). It is highly uncommon to go below a PTH of 70, though it can happen against targets that are significantly higher level than you.”
So having an enemy PTH of 60 - means the damage that hits will be 60/70 = 85.7% (or I got this completely wrong - as I just assume that the enemies play after the same rules and the damage reduction is not only applied to the player - but even than, there is at least the miss chance). But in general, having such a PTH vs nemesis (and factoring in debuffs which reduce your DA) requires a rather high investment in DA - which in general can be better invested in damage. Even this character can not always reach an enemy PTH of 60.
Edit: you can play with this a bit 🧮 Critical Hit / Resistance Calculators - DPS multipliers (low OA damage penalty)