If it's just for personal use and monitoring relatively differences, you can just pick one of the default calibration values. The AQ5 gives a "High flow", "5.6mm", and "3.3mm", but when I tried high flow with an BP 90 elbow at the outlet, I noticed it was reading about 20% low compared to my king flow meter. Since I already had the king meter, I simply went in and adjusted the pulses/liter until the two matched. If you didn't have another meter you trusted, you'd have to do some sort of flow test using a pair of 5 gallon buckets or something similar to that and measure how long it takes to transfer the water across and what the meter tells you.

But, that's only if you really wanted it accurate. I just know that barbs vary quite a bit on what their actual ID is and things like elbows would influence the result too. Not a big deal for general info and relative changes in flow rate, but I wouldn't take the number given as having absolute accuracy without calibrating it and checking.