This is a follow-up to my post about level or experience limits in OD&D.
I decided that if I ever run Original D&D with just the Little Brown Books (no Supplements), or a retroclone of such with very "faithful" experience-points-needed-per-level charts, then I will probably use a single chart for all classes instead of the default separate tables. I don't like how wildly the XP charts vary between classes by default (as opposed to, say, BX D&D), making it hard to pick equivalent maximum level limits if so desired. Plus, in this variation of D&D I don't think the advantages of giving each class its own chart are great enough to be worth the added complexity. A unified table would probably be faster to reference and to explain to players, and it might make it easier to compare characters, since I don't think it will actually affect "class balance" very drastically at all.
Here's what I came up with:
If you don't want a max level of 13, and you don't want to keep extrapolating all three classes' original XP tables just to pick the middle amount, here's a quick-and-dirty thing you could try: keep taking the "middle road" in a different way by requiring +240,000 XP per level after that, as per the Fighting Man class. Probably inaccurate, but maybe good enough?
I also made a unified XP table for the retroclone Full Metal Plate Mail, as seen below. It turned out to be exactly the same as the OD&D one until level 12. Honestly, even if I'm running straight-up OD&D instead of a retroclone, I might still end up using this chart instead of the more "accurate" one above. None of us are getting any younger, so I'm not sure there would be any benefit to making players earn 1.5 times as much XP for level 13 when I'd be grateful just to have the campaign last long enough for even 600,000 XP to be within a single player's reach. This table also has the advantage of being easy to extrapolate past level 13 if one does not like my chosen level cap: just add 100,000 more XP per level.