Copy pasted from a forum, posted here.
Currently BAB stacks straight up, as a Fighter 5/Monk 3/ Rogue 1 you'd have a BAB of +7. What if instead different BAB tracks stacked with themselves instead? So full BAB stacks with itself, 3/4 BAB stacks with itself and 1/2 BAB stacks with itself; then you add the results together. So in the above example the above character would be at 5th level in full BAB and 4th level in 3/4 BAB thus having a +8 to BAB instead of the +7.
Yes, this is called " fractional bonuses", and they should be in the Core Rulebook. It isn't as hard as Unearthed Arcana presents it (particularly with regard to Saves—that chart is a nightmare).
Base attack is easy: poor = 0.5 per level, average = 0.75 per level, and good = 1.0 per level. Just add 'em up and you're good!
Now Saving throws are an area where it is much more important to use fractional BAB, after all, its pretty easy to min max your multiclassing to get a decent BAB. But a Brb2/Ftr2/Rgr2 with base Fort of +9 a Ref of +3, and a Will of +0?!?!? What the hell were they smoking in 2000?
This one's a little tougher for the math challenged.
Base save bonuses: poor = 1/3 per level, and good = 1/2 per level with a +2 at first level. Now it's a little tricky to track all your levels in poor and good saves, but the important thing is to only give the +2 for the first level in any class that grants a good save.
So, the example above would be like this…ahem:
Barbarian, Fighter, and Ranger all give good Fort saves, so that's easy; 6 x 1/2 is 3, but only one +2 (for the first level in Barbarian) = +5 base Fort save.
The Ranger is the only class of the three that gives good Ref; so 2 x 1/2 +2 (for the first level in Ranger) is 3 plus 4 x 1/3 is 1 and 1/3 (I know, steady now) with the total 3 + 1 and 1/3 = +4 base Ref save.
And finally Will is also easy as all three classes have poor Will saves; 6 x 1/3 = +2 base Will save.
So our Brb2/Ftr2/Rgr2 now has base saves of Fort +5, Ref +4, and Will +2.
Poor BAB: .5
Average BAB: .75
Good BAB: 1
Add those up to find your total BAB
Poor Save: 1/3
Good Save: 1/2
Add those up to find your base saves. Add 2 to each save that has a Good save from any class, one time.