So I have not found a really good article on Rested XP. So I am going to write a brief overview of my findings. It would seem that the Rested Percentage is gained equal to your current level. Instead of a set amount of XP points it goes on a percentage system. What do I mean by that well let me explain. From what I have seen with the rested bonus, if I were to log out on my level 49 and gain 3% of my XP bar in rested XP, then I switch to a level 10 class, I still only have that same 3% but now it is 3% of a level 10, not a level 49.
I would love to hear from anyone with their findings because the whole system seems a bit confusing. I can say it would seem that the cap for rested XP is 2 Levels.