XP or Level Limits for OD&D (and Full Metal Plate Mail)

This is a follow up to my "XP Limits > Level Limits" post. I was trying to apply that concept to Original Dungeons & Dragons, a.k.a. the Little Brown Books (without the supplements, in this case), as well as a retroclone called Full Metal Plate Mail that I picked up a while ago. Looking at Men & Magic, I realized that the experience tables only went to level 9 for fighting-men, level 8 for clerics, and level 11 for magic-users. The book provides guidelines for advancing characters beyond those levels, but it doesn't specify exactly how much experience is needed to reach those levels.

Full Metal Plate Mail has more extensive experience tables, going up to fighter level 12, cleric level 13, and magic-user level 18. However, according to a post at Delta's D&D Hotspot and another at Playing at the World (and the comments below those posts), Gary Gygax intended the experience tables to be extrapolated beyond each class' name level as follows: +240,000 XP for every fighting-man level past 9, +100,000 XP for every cleric level past 8, and +300,000 XP for every magic-user level past 11. (See the picture below.)

Literal Back-of-a-Napkin Math

Full Metal Plate Mail follows a different extrapolation. Thus, a good XP limit for OD&D might not work as well for Full Metal Plate Mail, or vice versa. The other "Little Brown Books only" retroclones I've looked at, like Delving Deeper and Swords & Wizardry WhiteBox, seem to have even more varied experience tables; FMPM at least follows the OD&D tables up to name level for each class. I don't mean to cast aspersions on any of these retroclones, though, because I'm not convinced the original XP tables are necessarily the best possible XP tables. And the matter of what is "best" here is going to be subjective, of course.

So that was one hitch in my plan to find a good XP limit for OD&D. Another problem was that the experience tables for the three classes differ drastically beyond name level, at least if you follow the advancement scheme Gygax seems to have intended. At 960,000 XP, a fighting-man is only level 12, and a magic-user is level 13, while a cleric is at a whopping level 16! For comparison, 600,000 XP in B/X D&D gets you a level 12 fighter, a level 13 cleric, or a level 11 magic-user. Using the fighter as a sort of baseline character, I don't want to set my XP cap at a number that will result in any other class being too far above or below the fighter in level. Also, the OD&D fighting-man seems overly "expensive" in my opinion, while the cleric seems too "cheap."

So in the case of OD&D-without-supplements and the aforementioned retroclones, I'm tempted to just go by the following house rule: All classes use the fighting-man's experience table, and the maximum level is 12.*

I'm not convinced anything of value would be lost if I implemented this change, and it's appealingly simple. Honestly, I'm not sure that the concept of having separate experience tables for each class adds anything enjoyable enough to the game for it to be worth the extra complexity. If I ever get lured by the siren song of designing my own OSR heartbreaker system like so many lost souls before me, I think it's a safe bet that I'll use a unified XP table for all classes.

I still like to think my whole "XP Limits > Level Limits" concept has some merit for Basic D&D and its derivatives, though. I'm hoping to test the idea in a future campaign and see what my players think of it. As for AD&D, I haven't really examined the XP tables in depth, yet. The same goes for OD&D-with-supplements. I guess it comes down to whether or not these games are better off with separate XP tables.

One last note: Below are some XP limits I considered for Full Metal Plate Mail, along with some extra ideas for house rules.

Any of these might still work. I'm just not sure any of them are ideal.

*Update on 6/21/2020: I'm now starting to think that level 13 might be a better maximum level for all classes, based on the attack and saving throw tables in the "Little Brown Books."