The home made flow bench is a little more involved than that. You need another U-tube to measure pressure drop across a known orifice size BEFORE the "stack" (the stack being the tube below your head). This is usually a tank with a plate through the middle one side connected to the vacuums the other side connected to the stack. The plate in the middle gets a hole through it of a known size. This will allow basic repeatability. Voltage changes and weather conditions (temp especially) will make a HUGE difference from test to test. With a second U-tube that measures the "pressure drop" across the hole in the plate you'll write down both test pressures (think of the tank pressure drop like a "baseline") your flow (or stack) U-tube's variances will be able to be compared to your pressure drop variances which means you'll get automatic (automagical heheh) self calibration from day to day. Then you need a speadsheet with conversion calculations that give you CFM based on the known "orifice" in the tank, and the unknown modified port...
The best part is its self calibrating just correct for temperature and barometric pressure. Or someone in the house running a washing machine on the same circuit as your vaccum(s) (you will end up needing more than one 6.5 HP vac, to pull enough inches with .5" valve lifts or higher, to compare changes at high lifts). How do I know this? I have two 6.5 HP shop vacs running my bench
At high lifts the pressure drop at the tank side will only be a couple inches at best unless you have a tiny port. Which makes accurate measurement of small changes harder.
You also need zero leaks anywhere, including something sealing the head to the bore adaptor (the plywood). I use modeling clay. A rubber tube for your stack will vibrate and flex you need totally rigid PVC. Fluctuations are usually leaks, even insanely tiny leaks will cause movement... So will voltage changes as someone turns on or off other appliances.