A rising resistance also produces a hyperbolic growth to your effective HP, but what does this actually mean? A pet with 500 HP & 50% PR could absorb 1000 points of physical damage, so we'd say it effectively has 1000 HP. Now, imagining the same pet with 75% PR, it would have double the effective HP, despite just having a 25% increase in resistance.
eHP values aren't the be-all and end-all, since there's also damage that avoids resistances, but they help a ton when comparing tanks with one another. In situations where one resistance is overwhelmingly more important than the other, we typically push said resistance-stat up to 3 to optimise eHP, but for more typical tanks, 2 for each is the default choice.

We use this formula for the calculation: eHP = HP / (1 - Res)