The Rise of Karzoug
Fractional advancement is only an issue for multi-classed characters, if you only have one class you can ignore it completely. Normally a multi-classed character adds together the base attack and saving throw bonuses of all their constituent classes to get a total, but that can produce some perverse results with fractional advancement is meant to correct.
Consider a first level wizard looking to take another level. At level 1 a wizard has a base attack bonus of 0. If the character takes a second level of wizard his new base attack bonus will be 1 (as wizards use “bad” base attack progression of 1/2 level rounded down). Alternately if the character takes a level of magus he’ll have a base attack bonus of 0 (0 for the level of wizard, +0 for one level of “medium” base attack progression of 3/4 level rounded down). By taking a more martial class with a better base attack progression the character ends up with a worse base attack! That’s not great.
Consider a third level fighter. His base saving throw bonuses are 3 Fortitude, 1 Reflex, and 1 Will. Alternately consider a fighter/1, barbarian/1, cavalier/1. This second character is also third level and each of his classes has the same saving throw table as the fighter (good Fortitude, weak Reflex and Will). Despite ostensibly using the same progression and being the same level his saving throw bonuses are 6 Fortitude, 0 Reflex, and 0 Will. Again, not great.
How to calculate Base Attack
Every class grants either fast base attack progression (1 per level), medium progression (3/4 per level), or slow progression (1/2 per level). Add up all the fractions and then round down and you have your base attack bonus. The character in the sample above would have a 1 base attack at 2nd level.
How to calculate Saving Throws
Each class grants either good progression (1/2 level rounded down) or bad progression (1/3 level rounded down). If you have any classes which grant a good progression in a save you also get plus 2 to that save. This produces the exact same effects as the Core Rulebook for single classed characters. The example fighter/barbarian/cavalier above would have a 3 Fortitude (.5+.5+.5 rounded down plus 2) and 1 Reflex and Will (1/3 + 1/3 + 1/3), i.e. the same saves as a fighter/3.
Seriously? Fractions? Can’t I just do it the easy way in the book.
If the above seems hopelessly arcane, just give me your character and I’ll tell you what your BaB and saves are.