And then subtract 1 per class of armour (light, medium or heavy) that the character is wearing to a minimum of 1. Knights in full-plate armour tended to find shields pretty pointless for anything other than jousting.

(And recreated battles tend to overrate shields because no one tries to chop through the shield and few use their armour to deflect minor blows.)

But I agree that shields are seriously underrated in D&D.