For what it's worth: I have both the Allagan and Onion shields now, as well as full i90 (not the Allagan Blade yet though sadly, just Curtana Zenith!)
In standard PLD +VIT-build I90, at Seawolf base stats (no points allocated, just gear) I have 360 STR.
+ The amount mitigated per parry is 23%.
(This rises to 24% per parry at precisely 364 STR, NOT 363 STR as stated on Valk's site!)
+ The amount mitigated per block with the Allagan Shield is 27%
+ The amount mitigated per block with the Onion Shield is 20%
Adding 27 STR via accessories does nothing more to these numbers. Unfortunately I don't have any accessories with higher STR on them at the moment, but I'm fairly sure that the next Block/Parry strength tier is 405 on a PLD, which should JUST be achievable with i90 armor/weapons and HQ Gryphonskin Accessories for a Seawolf (and Highlander).
[Note that the i95 Allagan Blade that drops from Turn 5 grants another 2 STR over the i90 options, which would make hitting this cutoff point achievable for a Hellsguard, Seeker of the Sun, Duskwight and Midlander. Wildwoods, Keepers of the Moon and Lalafells will be just short of this cutoff in current gear...]
I'm not suggesting that swapping +Vit for +Str accessories is the way to go... just stating what's "possible"![]()
Technically a Highlander PLD with the Allagan Blade and a full endgame VIT build (363 STR) could invest 1 point of stat allocation into STR and get another +1% parry mitigation. That might just about be worth the tradeoff in ~14.5HP that 1 Vit gives you... but I'm fairly sure that we'll all be able to cross that STR threshold anyway without any stat allocation once i95 gear comes out!
Regarding Block/Parry rate... the Dex and Parry stats, and the Block rating values on the Shield all play a part in this. Sadly it's random and unreliable, but a ballpark average figure is about 20-25% Parry rate over time, with Block rate usually a few percent higher (for the Allagan Shield). I still need to test the Onion Shield for a while before making any sort of Block Rate comparison to the Allagan one... but a VERY quick test tonight suggests that it does indeed block a fair bit more often... I'm just not yet sure that the 7% lower mitigation when it blocks is worth the tradeoff.