One of the things I’ve found the most tedious about calculating experience in AD&D is the per hit point part of the creature’s value. I understand why a creature with more hit points is worth more than one of its fellows with fewer, but what a pain. A huge deal? No. But I don’t think it adds enough to the game to be worth it.
So I’ve decided I’m going to shortcut it. Rather than doing the calculation for every individual monster, I’m going to calculate out the value of one of each with average (4.5 per HD) hit points and use that for every copy encountered. I roll out HD normally, so the results should be perfectly fine.
For example, three gnolls I just rolled have 11, 6, and 11 hit points. The listed XP value is 28 +2/hp, so they should be worth 50, 40, and 50 XP respectively, a total of 140. Using my method we have 2 XP per hit point TIMES 4.5 hit points per HD TIMES 2 HD PLUS 28 EQUALS 46 XP per gnoll for a total of 138.
Another example: Gray ooze (3+3 HD) is 200 + 5/hp by the book. The gray ooze I just rolled has 16 hit points, so it is worth 280 XP. My method is 5 XP per hit point TIMES 4.5 hit points per HD TIMES 3 HD PLUS 15 (for the three extra hp at 5 per) PLUS 200 EQUALS 282.5 or 283 XP.
I’m making a list of monsters as I use them and adding the value to Appendix E in the DMG, though I might write them into the Monster Manual as well. This is an example of what I mean when I say we’re trying to play “mostly-by-the-book”: We don’t want to change anything if we can possibly help it, and when we change or houserule something it’s going to be with as little distance from BTB as we can manage.
Note: Sometimes, such as with the gray ooze example above, you end up with a half XP. I always round this UP in favor of the players. But then I’m a softie pushover DM like that.