[sammyg2]

Here's how I would do it:
Forget the small pipe. Build as flange into the larger pipe and install an orifice plate in that flange, small enough to restrict the flow slightly and cause a pressure drop across it.

then drill and tap the pipe on either side of the orifice and plumb up a delta P gauge (differential pressure gauge). Use the gauge to measure the pressure drop across the orifice, which should be reasonably linear until it starts cavitating.

You may have to play with the size of the orifice and range of the gauge until you can get both to work together, and you'll have to do some calcumalations to determine calibration, but it will be reliable, cheap, accurate, and kind of fun to do.