Just use the UA fractional tables, and you're done. Even if it was just "showing how the save progressions work," it's right there to implement if you want. And if they were just showing how it worked, they could have just said "2+ 1/2 level" and "1/3 level" and be done. The fact they showed a whole table of fractions means to me that they intended it to be a houserule to use those fractions.
Anyway, I would never allow fractional saves without fractional BAB, or vice-versa. Medium BAB multiclassing really gets screwed over with the normal rules. In general, fractional saves is a nerf and fractional BAB a boost, so they balance each other's inclusion. Of course, everyone's talking about the obscene good save stacking from the
RAW. No one's mentioning that bad saves get even worse...
A lot of times, you don't even really need special rules. If it's a rogue going into a "rogue prestige class" that's also medium BAB / good reflex saves, for example, you can just keep using the BAB and save progressions of straight Rogue 20. No need to even waste a second thinking.
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