Alea Iacta Est

The rules for epic characters (which have now made it into the SRD, to an extent) say that the first twenty levels (or HD) contribute to the character's base attack bonus and saves, and then the epic progression takes over. However, this means that a fighter 20/wizard 20 gains a full attack bonus then goes on to learn a full complement of spells, but a wizard 20/fighter 20 learns only feats from his fighter levels and does not improve his combat, being forever stuck with minimum attack bonus. Although the FAQ to the ELH specifically confirms this, I do not think players should be penalised for the order in which they took their classes (except for the differences at first level, which are not affected by this house rule).

My rule is then that the best twenty levels count toward base attack bonus. Figure this as follows:

Count the number of levels you have in a class that gives the good attack bonus progression (or HD of dragon, magical beast, monstrous humanoid or outsider). If you have twenty or more, then your base attack bonus is +20, and you've finished.

If you don't, take the number of levels in classes with the middle (cleric) progression (or HD of anything else but fey and undead). Only add enough cleric-equivalent levels/HD to make your total 20; discard any extra. Now add ¾ of your cleric-equivalent levels/HD to your fighter ones to get your total base attack bonus.

If you still haven't accounted for 20 levels/HD, throw in enough class levels with the worst (wizard) base attack progression (or your undead or fey HD) to make 20. Add half these to your total.

After you have the total, derive extra attacks from it as normal, at a rate of one per -5.

In each case, round normally as you divide things. This may not get exactly the same numbers as you were expecting, but everyone in my campaign is using it, so it is fair. If by carefully juggling your classes you managed to get a better base attack out of your first twenty levels than you would with this system, take it; you still won't be any better than a 20th level fighter.

Example: Möbius is a fighter 9/monk 8/sorcerer 8. He has 9 levels with the best progression, so he makes a note of 9 levels spent and attack bonus +9. Then he has eight levels of the middle progression, which can all count since he has 11 levels left to account for. He marks off 17 levels and base attack of +15 (9 + ¾ of 8 = 9 + 6 = 15). Then he takes three of his sorcerer levels to make up 20; adding their base attack of half a point/level he gets 1 point of base attack, for a total of +16, which expands to +16/+11/+6/+1 as normal.

As for saves, I have not yet decided how they should work. You may be stuck with your first twenty levels of saves, or more likely you can choose the best twenty of your levels. Players who take dozens of classes to get the extra +2 from the first level of the high progression will be penalised by being made to stack the levels rather than the bonuses. In any case, you must use whole class levels rather than individual saves; under no circumstances will a fighter/wizard be allowed to take the good Fortitude from his fighter levels and the good Will from his wizard levels, rather he could have the good Fortitude on half his levels and good Will on the other half.