Head = P / Density * g
From above value of H , you can easily get the flow rate from pump charactistic curve.
Please ensure that the Impeller diameter is shown in the curve is physically matching with the pump impeller diameter of which you want to calculate the flow rate.
Pressure gauge or any other transducer you are using for flow measuring must be properly calibrated .
No, you can't consider it to be the total head pressure. The TDH (total dynamic head) shown on the pump curve is the pressure rise across the pump (from suction to discharge), so you will need to also know what the suction pressure is, or estimate it. Note that the suction pressure sometimes is less than atmospheric pressure.
There is also the matter of accounting for velocity head. Typically the suction side of the pump is a larger diameter than the discharge side. However, many pump manufacturers already account for velocity head and include in the TDH curve, so you don't have to account for velocity head. You should check the curve or with the manufacturer to verify that is their definition of TDH.
Once you know the pressure rise across the pump, you can use the pump curve to estimate the flow rate. That assumes that the pump is running at rated speed and (and this is a biggie) that the impeller is in good shape. If the impeller is worn (say due to running raw gritty water through it for a few years), the pump will act as if it has a smaller-diameter impeller (at best) which means that for the same measured pressure rise, the flow rate will be lower. The pump curve shrinks toward the origin.
All the pump "sees " is the discharge pressure right at the pump outlet. Assuming you are measuring the pressure near the pump discharge then that is close enough for most engineering work. As you said if you convert that pressure in to feet of the fluid being pumped then you can get a pretty close estimate of the flow rate from the performance curve.
Remember there is probably a bit of an error in the pressure reading. Also unless the performance curve is based on a lab test for that exact pump there can be some variation in your pump's performance. Most pump manufacturers test just one or two pumps and then use that curve for all pumps of that size and design.
Assuming the pump is piped to the discharge point at the same diameter as the pump's outlet;
you will have to add pressure drop for the length of the piping, and equivalent length for bends in
the piping to head loss drop.
The head loss for total equivalent length will be proportional to flow rate.
How can you estimate flow rate using a pump performance curve given that you only measure the discharge pressure of the pump. I am also not sure if i can automatically consider the discharge pressure ( converted into head ) as the total head presented in the pump curve.
