Can't use real parse numbers as they vary with RNG. Instead you need to use statistical chance. 25% was just for easy math. If your parry always reduces 25% of damage then the difference between 20% and 25% parry rate is 1.25% average mitigation. Therefore, reducing parry by 1.75% would result in around 0.4% more damage taken. That doesn't seem like much... Until it is. Sometimes that sliver of mitigation is the difference between catching a heal or not. I just don't see how increasing the damage output of a tank with a -20% damage penalty is worth it. If that smidgeon of damage is significant then so must equal parry. Especially for the lower hp paladin, block and parry smooth incoming damage and help avoid the tanked being spiked to death.

I could be wrong, but I've always followed the tanking axiom of mitigation first. Just my two cents