I've brought up shields on this blog before in the past most recently in Splintering shields by Material a fiddly variant of Trollsmyth's shields shall be splintered rule, I also posted variants back in 09. Shileds are just so darned prevalent in warfare and underrated in D&D and many an OSR game I'm always finding myself whipping up variant rules for shields. Here's another based on a recollection of a method I used back in the late 80's.
Add a combatants STR, DEX and Fighting Rank (fighter level equivalent in most games) and check this score on the chart below.
(Click for larger image)