pHaestus
you're digging yourself into a hole
for the theory get an intro text to Fluid Mechanics
for a more practical run at it try Crane's Flow of Fluids Through Valves and Fittings
hit Google on 'flow resistance' and 'differential pressure'
here is a pdf of a educational test rig for exactly this subject
there are
2 parameters you need to measure:
the flow rate
the pressure drop (differential pressure across the item being tested)
and you'll crap your drawers when you measure the pressure drop across your flow meter
remember; each incremental flow resistance in the system is additive