No prob!
It's just a proportional thing. On this type of equation, the thing to ask yourself is: Is the flow 0 at 0 RPM? (yes) In that case, this type of equation (cross multiplication) will work great! Very accurate.
Let's say your pump flowed 6.63 GPM @ 8000 RPM and you wonder what it'll flow at 6600 RPM...
6.63 X
------- = --------
8000 6600
Multiply 6.63 by 6600 and then divide the result by 8000 and you get: 5.47
So, X = 5.47 GPM which will be the flow at 6600 RPM.
Let's say the calculator tells you that your system pressure will be 128 PSI at 8000 RPM, but you wonder what it'll be at 7200 RPM. First question: Is the pressure 0 at 0 RPM? YUP! Ok then....
128 PSI X PSI
---------- = -----------
8000 RPM 7200 RPM
(128 * 7200)/ 8000 = 115 PSI
Sound good?
Spud