Ideally, I think it would work best based off the block strength that comes on each shield, as it naturally goes up with gear as the cap. However, utilizing the difference in ilv between your shield and the monster to scale it's maximum potential. The higher the mob's ilv is above your shields ilv the less damage you would block, however if the shield's ilv is equal too or greater than the mob's ilv then it can block for it's maximum potential.

It would yield several benefits such as
-Giving PLD some actual mitigation advantage over other tanks, when equally/overgeared for content.
-The playing field would be level with other tanks when learning content, as shields wouldn't be as effective against higher ilv mobs giving PLD no significant advantage.
-It would give shields more inherent value again, making it a more viable option to upgrade early on when new content arrives.

Honestly, they could do something similar with that worthless Parry stat and increase it's ability to block/parry to something significant while using the characters ilv vs mob's ilv difference to scale it's maximum potential. At least it wouldn't be a totally dead weight stat when tanks are forced deal with it when upgrading gear.