[osr] shields shall be activated

Have the summer end rains already blessed your part of the world? I can feel them approaching in timid peeks; barely passing through the ranks of their mortal enemy, the ch3m7rails. Welcome to your comfy mood blog where we sometimes post and discuss rpg houserulings. 

I was working on a chart tonight. In my d6 pool d&d there is one rule that proved to be very cool: the shield roll. Shields basically do not add to your AC, but instead block an attack 1/3 of the time (roll a 5+ on a d6)

This means they mean much more protection than the small +1 (5%) they add to AC in B/X. But the increase is not good on itself: its a matter of taste. The good part is that it's relative importance increases as you wear less armor: the percentage of blows stopped increases as you are easier to hit. 

For example, a PC in plate armor can be hit on a 6 (17% chance), so the shield drops the chance by a third (by about 6%). The same PC in light armor is hit on a 5 or a 6 (33% chance) so the shield drops the chance by 11%. At AC four (no armor) and three (no armor and yet a level 1 fighter) the shield will be useful in 17% and 22% of the attacks received, respectivelly.

I made a chart that ports the system to B/X like this: Instead of adding a +1 to your AC, the shield can be activated once you have been hit (before damage). Roll a d20 when you do it: your AC becomes that number against that specific attack.
These are the chances against a monster with no attack bonus. Took a little time to calculate the percentages manually with the Windows Calculator, then I translated it to AC:

As you can see, the relative protection of the shield increases greatly, from their simple +1 ac on the original; being much greater in the lighter armors, but never too big as to make them useless. A fighter in leather and shield has 16 AC! just as if he wore plate. However, a one in plate and shield is just 18 AC, just one pip over his original equivalent, which I like because it mimics how plate armor got diminishing returns from the shield in real life.

Against monsters with actual attack bonuses (which is the norm) this shield boost diminishes gradually, but still getting on a median an extra AC point (around +2) at chainmail levels, with marginal benefits the greater the armor and the greater the monsters, but not reaching total zero. This way, the decision on wether or not to use one is always present.

YMMV about if the increased AC is a good thing or not. To compensate for it, I'd limit the shield activations to one per turn, which will only be relevant against monsters with multiple attacks or against multiple enemies.