Here is how I did it.
I put a measured 5 gallons in the tank, noted the level was not enough to even get a dip stick wet-----let it sit for five minutes or so, and rocked the plane vigorously. Let it sit for another 5 minutes, and rocked again. Then pulled the drain valve, and let all the fuel flow out, into a funnel with a coffee filter in it, then into the fuel can.
Got back a little under 4 1/2 gallons. That gave me a "unusable" fuel figure of slightly less than a gallon-----I just figure one gallon to give myself a pad, and for simplicity of math.
I got a lot of crud ----alum flakes, dirt, and proseal in the coffee filter.
Repeated procedure for the other wing.
Then, put half of the fuel in each wing, and did the flow test with the fuel line removed at the servo-----again draining into the funnel/coffee filter.
More crud, and a fuel flow of close to 40 GPH IIRC. Way more than minimum.
By doing it this way----wings first, then rest of the system, I insured the fuse plumbing would be supplied by clean fuel, and the fuel filter would not be overloaded at the first run----and the last of the crud should be what came out at the fuel servo. Seems to have worked as planned
Then, I took the used fuel and put it into my motorcycle, lawnmower etc.
When I actually filled the tanks for the first time, I made a dip stick for each 5 gallons added.
I have had no issues with the fuel system, other than a weeping rivet or two.