How To Measure Fuel Volume.

Free flowing into a bucket off the regulator tells you nothing useful.
Under those conditions the pressure after the regulator is zero.

What matters is how much does the system flows while maintaining your minimum desired pressure.

For that test you need to put a valve at the end of the line going into the bucket.

Then you adjust the valve with the pump running so the gauge reads the lowest pressure you want to see(around 5psi).

Shut the pump off, don’t touch the valve.
Empty the bucket.

Have someone turn on the pump and time how long it takes to pump one gallon.

It’s pretty safe to use .5 lbs per hp/per hr for figuring fuel consumption.
Figure gas is about 6lbs per gallon.

So, if it takes 90 seconds to pump 1 gallon while maintaining 5psi, that’s 40 gallons per hour, x 6 = 240 lbs/hr.
240lbs/hr divided by .5 = 480hp.

All that said, If your system is maintaining 6psi thru the finish line, it’s unlikely you have a fuel delivery issue.