The ones with "builtin" reporting (LCD etc) all tend to be WAAAYYYY off the mark, and restrict flow by a fair degree (ie: step down to a 4mm or 6mm jet within the flowmeter).
The best way to do it is to get a Swissflow SF800 (the best affordable and least restrictive one to use for PC Watercooling), and connect it to a MatrixOrbital or similar. The SF800 only drops down to a 10mm jet, so is the equivalent of having 1/2" tubing step down to 3/8" for 2" of length, then back up to 1/2". If using 7/16" tubing, then the restrictivity won't be noticeable.