I like the fill the drum idea, maybe 2 5 gallon buckets would be more practical?

Doesnt work like that with an open pipe, there is no pressure on it so that would be free flow. You must have a way to test pressure drop across the water coil and manuf. specs to calculate GPM PERIOD!!! Anything other than that is a guess at best. Remember the water outlet has pressure on it too. EVERY manuf. out there has these specs available.