Just a simple thought exercise...
I prefer using milestones instead of tracking XP in my games. With different XP tracks, this can become hard. The table below notes 55 milestones that guarantee that someone is levelling up whenever a new milestone is reached (provided there is a PC of the appropriate class; e.g., the 55th milestone makes a difference only for MUs).
It is up to the GM when to give the first milestone (thus getting the thief to level 2). After that, one milestone per dungeon or significant goal is appropriate. Or in whatever pace the GM wishes. The point is that no individual XP tracking is required.
Alternatively, if the GM wants to give away XP per session/dungeon/challenge, the table below suggests an appropriate amount. E.g., if the PCs have about 500,000 XP, give them 20,000 XP for the next milestone.
If you prefer a formula to a table use, the 10% method. Start with 100 XP and raise it to 200 XP after the PCs reached 2000, then 300 at 3000 XP, 400, 500, 600, 700, 800, 900, 1000, 2000, 3000, etc. The idea is to give away about 10% of the current (average) XP but disregard all digits except the first one: for example, if the PCs have an average of 4450 XP they get 400 XP per milestone (and not 445). This progression is granular enough to keep PCs leveling at different paces. Notice that all PCs get the same XP, roughly based on their average XP (and milestones should take this average into account - defeating a dozen goblins is no challenge for a party with 50,000 average XP).
Come to think of it, the formula is a lot easier than using the table.
Would I use this? No, I prefer using unified XP, as noted in my house rules:
Here is what I've suggested in Old School Feats: If you prefer unified XP tables, we recommend using the magic-user table for all classes, adding a few extra feats for Clerics, Thieves and Fighters. Thieves also get 1d6 HP per level, like clerics. Clerics need no further enhancements. This changes some assumptions but is still balanced in my opinion. If you prefer unified XP tables, we recommend using the magic-user table for all classes, adding a few extra feats for Clerics, Thieves and Fighters. Thieves also get 1d6 HP per level, like clerics. Clerics need no further enhancements. This changes some assumptions but is still balanced in my opinion.
Anyway, just a random idea. If it is not clear enough it is because I never used it; maybe someone else can develop it further.