Just make sure with a pen that you check it's accuracy, as nearly all of them are wildly out.
1. Pre prime a 0.5ml syringe with water, suck it up and squeeze it out, to moist the tube so it has less friction, and to prefill the needle.
2. Fill a cartridge with water
3. Stick the syringe into the end of the cartridge
4. Set some clicks and see what it shows as on the syringe
This way you can then calibrate your clicks to the real dosage.