Well, the training agencies set their own recommended PPO2 limits for RB diving. According to BSAC SDPs, these are 1.3 for the duration of the dive, 1.4 for deco.
Hopefully, there's some sense behind these, rather than just arbitrary limits
As for the PPO2 changing with respect to depth during the ascent, assuming you weren't consuming any O2, it would rise as you descend. Obviously, if the diluent were pure Nitrogen that was being added, the PPO2 wouldn't change at all, but as air is 21%ish, it would creep up as diluent is added. As I've never actually dived one, I can't actually answer the question from a practical perspective, only from a physics-ish one.
But I'll stop being an armchair RB diver in a couple of weeks when I do my CK MOD 1
