I'm sure this has been answered somewhere, sometime before, but my Google-fu is failing me.
What happens when a template is added to a creature with a fractional CR?
For instance, I need an advanced giant hawk. According to the Bestiary preview, a normal hawk has a CR of 1/3, and the Advanced Creature and Giant Creature templates each add +1 to the CR, for a total CR of 2-1/3.
What is the advanced giant hawk's final CR? Do I ignore or round down the original 1/3 and make it CR 2, or round up to CR 3?
You might also want to wait until the after then encounter and then see how challenging it actually was and assign XP appropriately.
Not always the best idea. If a party comes up with a unique strategy that makes the encounter ridiculously easy, they should get full XP to reward their creativity, even if the fight wasn't challenging. Conversely, if they fight like individual morons instead of a group of allies, and they get schooled by a simple encounter, they shouldn't get more XP because it was more challenging.A run of good or bad die rolls can similarly make an encounter seem to be more or less challenging than it should be.
I'd look at the resulting creature and compare it with other CR 2 and CR 3 creatures. Whichever one the stats are most similar to is the CR value I'd assign.
This is the idea I'd use :)
I can't remember where I read it, but, I seem to remember a suggestion that fractional CR creatures use CR=1 to compute their templated CR.
This is how I've also dealt with it; for example, if I use the Burning Skeleton (from the Bestiary Preview), it's a CR 1 monster. Whenever in doubt, always round down! :)