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.* Which is: Clerics require 25% less experience per level than Fighters, but round up to 100,000 xp at 8th level, when the experience required is within 25% of 100,000.
I'm also considering changing the 120,000 xp for 8th level Fighters to 100,000 xp, but I haven't decided if I should go that radical yet.
I can avoid forcing people to do math by preparing this table:
|Level ||XP Chunk Size||To advance to the next level:|
|2||1,000|| Clerics need 3 XP chunks|
|3||2,000|| Fighters need 4 XP chunks|
|4||4,000|| Magic-Users need 5 XP chunks|
Clerics, level 8: 100,000 xp total, +100,000 xp per level thereafter.
Magic-Users, level 5: 20,000 xp total, +20,000 xp per level to 9th.
Magic-Users, level 9: 100,000 xp total, +100,000 xp per level thereafter.
This basically does the math for you. If we pre-divide experience points for each Fighter level into fourths, we can just say that Clerics need three fourths and Magic-Users need 5 fourths.
I might want to use a different terminology than "XP chunk size", though.