I use the Dwyer VF series flowmeters (VFB-67-BV-PF-SPCL-VIT). $95 for an oxygen clean, viton o-ring version, and they make them in a bunch of different sizes. Supply it with a welding regulator and you can flow as much oxygen as you want with a great deal of control:
Series VFA & VFB Visi-Float® Flowmeters Product Configurator | Dwyer Instruments
Keep in mind that flowmeters are calibrated assuming that you're flowing into the atmosphere and the displayed LPM is only ballpark. You would need to be able to measure the output temperature and the actual pressure to get an exact figure. The bigger challenge to accuracy is that the compressor's input isn't flat and you'll need to adjust the flow slightly over the course of a fill to maintain your desired nitrox percentage. Or you can "set it and forget it", depending on how much your care about your fills hitting your desired nitrox percentage. I aim for within 2/10 of a percent because I geek out on the whole process. Many (including agency standards) are quite content if the final blend is within 1-2%.
Anyhoo...
At 5 CFM input, you need roughly 15 LPM for 32% and 27 LPM for 40%.
At 3.5 CFM input, you need roughly 11 LPM for 32% and 19 LPM for 40%.
Dywer makes flowmeters with 3-25 LPM scales, 6-50 LPM scales and 10-100 LPM scales. Most of us garage guys are going to want the 3-25 LPM version.
You can also order the Dwyer flowmeters with an arbitrary scale and not worry about the LPM display. It's more important that the bore is correctly sized to your compressor input. If you tried to use the 10-100 LPM flowmeter on a 5 CFM compressor, you'd have use a combination of low input pressure and low displayed LPM to get your desired 15 LPM flow rate to produce 32%. Fine adjustment would be very difficult, and your fine control would come almost entirely from the regulator pressure instead of the flowmeter. Better to use a 3-25LPM flow meter and gain a bit more accuracy.
On my compressor, 25 PSI on the welding regulator = 32% at 11 LPM displayed, and 40% at 20 LPM displayed. Pretty close to the expected flowrates, but more importantly, in the middle of the flowmeter's range and the welding regulator's most stable output pressures (15-40 PSI, per manufacturer). Adjustments are easy.
Have fun and don't blow yourself up.
-B