Since the original rules used six-sided dice for hit dice, regardless of class, the hit die progressions are not as straightforward as later editions and clones based on them. For one, some levels for some classes "skip" adding a hit die, adding a hit point instead. For another, the patterns aren't regular, and even the class that adds a hit die every level inserts bonus points at irregular moments:
Level HD Level HD
1 1+1 6 6
2 2 7 7+1
3 3 8 8+2
4 4 9 9+3*
5 5+1 10 10+1
* (Incidentally, the HD values for 9th and 10th level seem to imply that rerolling all hit dice when a new level is reached was, in fact, the default assumption, since it is impossible for a player rolling one hit die every time a level is gained and adding it to a running hit point total to get 11 hit points at 10th level, even though this should be the minimum; the minimum for 9th level is 12 hit points, so it's possible for a player who rolls badly to actually lose a hit point when reaching 10th level.)
I don't think I want to clone the LBBs so perfectly that I keep the 8th and 9th level Fighter HD at the by-the-book values. I'm willing to let those go. A simpler solutions is:
Fighters have one hit die per level up to the 10th; on the 1st, 5th, 7th, and 10th level, their hit points are equal to (level) HD +1 point. After the 10th level*, they have 5 HD + (level)/2 HD, round down, +3 points on odd levels or +1 point on even levels.* (Another interesting observation: by the rules, there is no hit die cap in OD&D. A 20th-level Fighter would have 15+1 HD, by the book. I don't know that anyone uses the LBB progression above 10th level.)
Magic-Users add a hit die every other level -- for a while. Rather than struggling to keep warlocks at 5 HD, I'd rather keep the every other level rule:
Magic-Users have (level)/2 HD, round up; on even levels, their hit points are equal to (level)/2 HD +1 point. After 19th level, they have 5 HD + (level)/4 HD, with the remainder converted into individual hit points plus 1 point.This means that MUs fall behind the LBB progression from 8th through 11th level, then starts catching up, passing the LBB progression at 21st level. They never have as many hit dice as Fighters, though; a 20th level Wizard has 10+1 HD under this scheme, while a 20th level Lord has 15+1 HD.
Clerics add 1 HD every level, except for a little "delay" at 5th level, when they get 4+1 HD. They fall behind Fighters at that point, but stay ahead of MUs. After name level, they add a hit die every three levels. We can get reasonably close to the LBB progression with three formulas instead of two:
Clerics have 1 HD per level up through the 4th level; afterwards, they have (level)-1 HD, plus 1 point if 5th level. After 8th level, they have 4 HD + (level)/3 HD, with the remainder converted into individual hit points plus 1 point.I've said previously that I'm OK with the Target 20 approach for combat ability, but that really only emulates the LBB for levels 1 through 12 or thereabouts, and mainly only for Fighters. Target 20 assumes that 1st level Fighters have a better chance to hit than 1st level Clerics or Magic-User; but in the LBBs, everyone, even "0-level" normal men, start the same. A simpler solution is to link combat ability to hit dice instead of levels, halving hit dice for Clerics and Magic-Users, but guaranteeing that the combat bonus is always a minimum of +1.