There's not going to be an offset from what you measure; the difference will depend on the amount of power that the CPU puts out.
I don't think I'd be able to calculate any offset either; the calculation is going to depend on a lot of variables.
I'd take a guess at (load, 75 W) 15 deg C.
Check out this thread for some info:
http://forums.procooling.com/vbb/showthread.php?t=9445