Some progress...
I had to chart the PQ curve for what I assume is your pump: Danner MagDrive 3.
I then used the pressure drop graph from Bill's radiator roundup, for the Big Momma, to calculate the remaining pressure available (for the block).
Observations:
#1: that pump has an optimal power range in a low pressure setup, and that range happens to be way outside what you can expect in a loop/rig for a PC. How attached are you to that Danner pump?
#2: Here's the data I've put together:
Pump pressure at 4.75 gpm: 1.5 psi (red)
Pump pressure at 3 gpm: 2.6 psi (yellow)
Pump pressure at 2 gpm: 3.0 psi (green)
Pump pressure at 1.5 gpm: 3.3 psi (dark blue)
Pump pressure at 1 gpm: 3.6 psi (light blue)
Radiator pressure drop at 4.75 gpm: very high
Radiator pressure drop at 3 gpm: very high
Radiator pressure drop at 2 gpm: 1.6 psi
Radiator pressure drop at 1.5 gpm: 1.0
Radiator pressure drop at 1 gpm: 0.3
Remaining pressure at different flow rates:
2 gpm: 1.4 psi
1.5 gpm: 2.3 psi
1 gpm: 3.3 psi
Given that the power is calculated as dP x flow rate:
2 gpm: 2.8 (unknown units)
1.5 gpm: 3.45
1 gpm: 3.3
So my conclusion is that your target flow range should be 1.5 gpm, and preferably closer to 1 gpm, if not 1.5. This will give you the maximum amount of "hydraulic power" for your nozzle.