A way to "block" attacks with shield equipped

Some retro games allowed shields to block a certain percentage of damage, usually based on  a defensive attribute, item quality (AC?) and skill level (tactics?). I think allowing shields to block a small percentage of PvP or PvE damage could make them more useful and add a certain dynamic to the game. Just my two copper.

Sorta how helms (used to at least) prevent PvP crits.