My SSI book has the spot for SAC in psi/min. The equation given for cf/min includes volume so that rate should be the same in any given size tank, correct?
That's in the deep diving SSI book. The calculations are done in psi/min considering an AL80 tank.
In the Luxfer web site (Luxfer: SCUBA Cylinders: Specifications) it is mentioned that the Al80 tank has 77.4 cuft of capacity.
If you consider your SAC in cuft/min you will independize your calculation of the tank size. If you are using a tank of your own and will always use the same tank, you could go on using SAC in psi/min.