Putting a standard hose on the return fitting will increase head loss significantly so the flow rate will not be very accurate.
I can estimate the flow rate for you but I need the following information:
Suction pipe diameter (e.g. 1.5", 2", 2.5")
Average length of suction pipes
Number of individual suction pipes from the water to the equipment
Number and type of suction side valves
Current Pump Make/Model
Height of pump relative to the water level
Pipe size between pump and filter (e.g. 1.5", 2", 2.5")
Filter type, size and multi-port valve size
Filter PSI reading at the desired valve settings and pump's highest speed setting
Height of pressure gauge relative to the pump center line
Heater - yes or no
Number and type of return side valves
Return pipe diameter (e.g. 1.5", 2", 2.5")
Average length of return pipes
Number of individual return pipes from the equipment to the water
Return eyeball/jet inner diameter (e.g. 1", 3/4", 1/2") or Jets (e.g. 3/8", 7/16")
Number of return eyeballs
Optional: Pump suction reading (in-hg) - not necessary but improves accuracy