For the flow test, I use a ball valve on the end of the hose going into the jug.
I adjust the ball valve so the system pressure is the lowest I’m comfortable with........ like 4.5psi.

Then, don’t touch the valve....... empty the jug, and do the flow test.
You want to see how much the system flows while maintaining the required minimum pressure.

For 750hp....... the flow @ pressure should be slightly better than one gallon per minute.

The free flow test at zero pressure tells you nothing useful.

We tested a system in a car once where the free flow was great, but the flow at 6psi was barely a dribble.