Stupid, Simple Way to Convert Hit Die to BRP Skill Level

The DIY-OSR scene has the best adventure content but I like to play with Chaosium's BRP brand of d100 mechanics for the systems approach to combat and skills. D&D and BRP have an identical approach to Character Attributes, Hit Points are close enough to not matter, but Armor Class and the Monster Attack Table don't port over to the BRP d100 resolution mechanic without some thought.

Here is what I do; the Monster Attack Table is the easiest to convert out of the two. Armor Class 9 (or 12 for ascending) is an unarmored opponent. WarHammer Fantasy Roleplaying has your basic scrub getting a 35% to hit, so for a 1 HD Monster chance to hit is 35%. I'm going to grab the Monster Attack Table from Mutant Future and just add 5% for each row of Hit Dice which improve the character's chance to hit. So a 1+ and 2 HD creature has base line combat (or any primary skill/ability) of 40%. 2+ to 3 HD creature is 45%, etc. Following this progression you top out at 75% chance to hit/primary ability at 9+ to 11. OSR Creatures/Characters at these higher HD will most likely also have some unique attack abilities which will compensate for the flattening out at 75%. But that is all I have been doing for establishing a relation between Hit Dice and a Primary Skill Ability. 

AC is not as easy. Armor Class in D&D represents how hard it is to not only hit your target but to hit decisively enough to actually deliver damage. BRP is less abstract. How difficult one is to hit is not tied to how much damage one would necessarily receive if hit. This is where the creature/monster/character description is important. Not that it shouldn't be anyways regardless of system. I bring it up because as a gamer I can't but help and look at NPC's as just a pile of stats so I can adjudicate action quickly. I find trying to stat creatures out for another system makes me look for what makes the Leech-Man different than the Kobold. 

So looking at the common Kobold. With AC 7 a 1st level character(scrub) needs a 12 or better to hit, or 40% or less. The description of your average Kobold (from B/X) leaves me to believe they don't wear any armor so their improved AC over unarmed will come from their small stature and quick movement. Based on their HD their base combat ability would be 35%. Makes sense to me. Their improved armor class will come from an improved Dodge skill. Say boost 35% up to 45% (5% per each AC better than 9). HP stay the same. So your average OSR B/X Kobold quickly translates into Attack/Parry skill of 35% with a Dodge of 45% and no armor protection to reduce damage if hit!