I did some digging, from a guy on reddit that did tests it took about 2.6 DEX to equal 1 parry stat. I had DEX-melded some heavy wolfram, with 21 parry and 13 DEX on the helm it was effectively the equivalent of 25 or 26 parry.

Parry's not horrible for dungeon running though, more small hits you're taking the more it'll even out. But definitely for endgame content don't go out of your way for it.

And as far as DEX rings for PLD because of parry/block rate, STR boosts the damage lessened by those parries and blocks so that argument's a moot point. Do not wear DEX rings on PLD. The only time you'd consider DEX on accessories is if you were so keen on parry despite what everyone's said, that you'd get topaz accessories and meld STR, then DEX. Even ruby you'd meld VIT, then parry before you'd even consider DEX. Which, bringing it all back to "parry is a bad stat", you wouldn't.