[John Rogers]

I would suggest measuring how much power the motor driving the pump is using and that will equate to how much water is being pumped. Having the small line off the side of the main line will never give a reading relating to the actual flow. You would need some restriction in the main line to force the water through the small line and then it would not match actual flow.

I would suggest installing a tee and short section of the 2 1/2 pipe and a valve that opens with no restriction down stream of the filters, etc. A valve in the main line would also be needed so you could shut off main flow and open the test line to see how long it takes to fill a 5 gallon container. This would be pretty close to actual flow rate.