If you piggyback the PS pump off the alternator pulley so long as both groves on the alternator pulley are the same it has no effect on the math.
The PS pump will turn just as fast or slow as it would if it were belt driven directly off the crankshaft pulley.

I had some difficulty with the fan belt wanting to slip on the alternator pulley with the setup you describe. If the alternator pulley were bigger that would help..

Grigg

•1951 GMC 250 in the Project Journals
•1948 Chevrolet 6400 - Detroit Diesel 4-53T - Roadranger 10 speed overdrive - 4 wheel disc brakes
•1952 Chevrolet 3800 pickup
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"First, get a clear notion of what you desire to accomplish, and then in all probability you will succeed in doing it..." -Henry Maudslay-