To me XP should be simple, not horribly Complex.
Say Fast XP is 100 XP to go up a level. Have it simple in that a CR=APL encounter is 10 XP. An APL+X encounter is 10+X. An APL-X encounter is 10-X. This allows you a bit easier tracking and it allows room for Creative Solution XP.
Like my Rogue figuring out a way to by pass an ambush I setup that they find while scouting. She gained a little bit of experience over what the encounter normally would have given for the solution of moving through the forest to get to the Army to Ambush the Ambush.
I would have easily given her 1 or 2 XP in this system instead of worrying about a Fair amount to not get accused of playing Favorites. Though it didn't help in the end as she was by far the Smartest of the Players... literally she is the only one that didn't need a Calculator to do something as simple as 20+12.
Thank heavens I only needed it when I was overwhelmed with numbers. Nothing beats a Dragon getting five Bonuses all that get added to something that already had three numbers to Add to ghetto large number of Dice I had to roll.