It's actually pretty easy to figure out mathematically. For BAB, good BAB classes get +1/level, average BAB classes get +0.75/level, and poor BAB classes get +0.5/level. Always round down. That's it! I forget what they give as the numbers for Saving Throws or if they account for the 1st level "bonus" for good saves.
So, using these optional rules, a Wizard 3/Rogue 3 would have a BAB of +3 (0.5+0.5+0.5+0.75+0.75+0.75=3.75, rounded down to 5). The same character under the PHB rules would also have a +3. I think it only really starts to help at higher levels.