Can someone explain RAC for me? How is it calculated?
http://boinc-wiki.ath.cx/index.php?title=RAC
RAC(new) = RAC(old)*d(t) + (1-d(t))*credit(new)
Where d(t) is the decay function, and t is the time (in seconds) since the last Recent Average Credit recalculation.
d(t) = e^(-ln(2)*t / 604800)
The Recent Average Credit for any Computer, Participant, or Team is calculated every time that Credit is added to that object. Please see also the example on the right.
Breif section from the wiki
HTH
It does… i think! I get it, but I was hoping it was somthing I could quickly figure in my head… guess not! Thanks Drezha! 
If you use the analogy of a car, your total credit is your odometer, your RAC is your speedometer - an approximation of the speed at which you are producing credit.