Chris. About the only way you can accurately flow test this system is with a flow meter inline. Once you disconnect the hose from the inlet manifold you reduce the "system load ie. FPR won't be controlling.

See if you can borrow a CODA Fuel System Analysis kit. It has a flowgauge that connects in series with the fuel line.

Another way I have measured flow on returnless systems is using an adjustable FPR. I have measured pressures and set scale marking on the adjusting screw. Disconnect the fuel hose from the fuel rail and attach the adjustable FPR. Set it to just under the system fuel pressure ie. if system fuel pressure is 250 kPa, set the adjustable FPR to 225 kPa. Run the pump and measure the output on the adjustable FPR's return/output line.

Its not perfect but its close.

Generally, internal pumps (if roller cell) expect a flow of 1.5 plus litres/minute.

If internal "fluid friction" or gear pumps expect a flow of 0.85 plus litres/minute

HTH

Peter