@NutjobBob @1Lysander Don't take this the wrong way, but your posts are rather confusing and it took me a while to understand what you're trying to say. I don't think Crafty and Ven have quite understood you either so I'm going to attempt to explain what you're suggesting a bit clearer.
In a nutshell, make the xp bank work the same way as the players level bar.
When a player wants to store xp calculate the approx. number of experience points they have and add it to the number in the bank.
E.g. Player has 7 levels, 7 levels = 91 xp
Bank has 10 levels, 10 levels = 160 xp
New balance = 91+160 = 251 = 13 levels (round down)
When a player wants to take xp out do it as levels.
E.g. Player has 0 levels, bank has 13 levels
Player takes out 7 levels
Player then has 7 levels, bank has 13-7 = 6 levels
This avoids the two apparent problems with just using exp or levels that other people have pointed out above. Namely:
If only using levels; Players can repeatedly bank low numbers of levels, which require very little xp. Letting them accumulate very high numbers of levels much faster, and by collecting much less xp, than would normally be possible.
If only using xp; Players can bank very large amounts of xp and use that to repeatedly gain low levels (i.e. go from level 27-30). Although this is less problematic in my opinion because it's no different to using an xp farm until you hit level 30, enchanting, and then repeating.
Unfortunately the method @1Lysander suggested still has issues. With a bank like that you could essentially farm xp from it by storing small amounts of xp and taking it out out again while you are at a much higher level.
E.g. You bank 27 xp = 3 levels
Level up to 27, you can then use the 3 levels to go from 27-30. That would normally cost 310 xp, but you just did it for 27!
Rinse and repeat
Obviously you could work around this either by forcing players to bank all their xp before taking any levels out, or by preventing people from taking out levels if they have more levels than are in the bank but by this point no-one is actually reading this god-awful absurdly long post anyway it's probably more trouble than it's worth to implement.
K, bye
that explanation was pretty good but I did say to avoid the "27-30 levels with only 3 in the bank" by making it so that in order to take levels out you have to have less than or equal to the amount of exp in the bank before and after the transfer.
e.g.
you have 31 levels in the bank and 27 levels on you,
you can take 2 levels out of the bank before you cant take any more.
this leaves you with 29 levels on you and 29 levels in the bank