Experience points needed for each class is pretty easy up to 5th level. Fighters need 16,000 experience points to reach 5th level, half that for 4th, half again for 3rd, and half of that for 2nd. Magic-Users require +25% experience for the same level, and clerics require -25% of Fighter experience for the same level, which means MU and Cleric experience doubles each level as well. After 5th, Fighter experience minimums continue to double each level up to 8th, although only two significant digits are kept (so superheroes need 120,000 xp instead of 128,000 xp.)
Magic-Users and Clerics get pretty wonky after 5th level, though: the goal seems to be to make sure that 9th level MUs and 8th level clerics require a nice round 100,000 xp, half that for both at 7th level. For MUs, the experience points for even-numbered levels above 5th are midway between the xp requirements for the odd levels; for Clerics, the 6th level xp requirement is half that for 7th level, which means that there's an extra thousand points to make it to 5
The challenge is to describe this simply without duplicating the LBB chart, which is forbidden. Should we say (Option 1):
Fighters need 2,ooo experience points to reach 2nd level; every doubling of experience thereafter adds another level, up to 64,000 xp at 7th and 120,000 xp at 8th, plus an extra 120,000 xp per level thereafter. Magic Users require 25% more experience, and Clerics require 25% less, up to 5th level... And then define the experience needed on a per-level basis thereafter? Or do we stick with the +/- 25% formula until they break 100,000? For the latter, the results are (Option 2):
This raises experience for both classes, although that doesn't seem out of keeping for the Magic-User... but Clerics now require quite a bit more experience per level after the 9th. Another option is to round up to 100,000 when experience requirements get within 25% of that value (Option 3):
This delays the cleric's anomalous jump at 6th level, but otherwise keeps it close, but still slows down the MU's advancement considerably, compared to the LBBs.
Probably the best answer is to go with Option 3 for Clerics, but for Magic-Users switch to +20,000/level after 5th, up to 9th level, then +100,000/level thereafter. This means that only 6th through 8th levels have incorrect experience for Magic-Users.