i agree with jersey, 3tenths is a lot to get out of a belt. there is no real load on the ps pump when going in a straight line, it won't build much pressure until you steer the car.
if your considering it you may wanna jsut go to a pinto style manual rack and save yourself the trouble of swapping those belts all the time anyway as well as the 25ish lbs of the pump and all the fluid
later
tim