Flow & Pressure are two different beasts.
If you have mains pressure of 4 bar then you can push that water up 40 metres, but once you take in other factors, pipe runs, valves, etc, by the time the water reaches the tap, you might be down to 3 bar.
You can have high pressure, but low flow, you may notice this when you turn on more than one tap elsewhere in the house.
To get a accurate flow rate, it ideally need to be done from the rising main.
Your looking for a minimum of 2 bar, but not over 5 bar.
EDIT: Water pressure can be measured 3 ways, BAR, PSI & Metres. 1 bar = 10 metres Head = 14.5 psi.