To calculate for head loss, you need to hook your hose up to the outlet/nozzle wherever it ends up. Then you would know what that particular outlet was rated at. You would need to test all outlets (separately, I suggest!) in a tank to measure the water flow in that tank. All outlets do not necessarily have the same flow.
If you just want to measure the pump without calculating head loss, just hook the hose up to the outlet on the pump itself.