In GCC or on the connect app. Go into the doser section. Then select which doser head you want to calibrate. Then find "Calibrate the pump now". You will need some way to measure the fluid, like a graduated cylinder. Hit calibrate the pump now, it will run for 1 minute while you collect the sample from the end of the dosing line. Once finished, read how many ml were dosed. Enter that number in the "Flowrate of the pump" box in GCC or the app. Then click save.
You can do this a few times and average the ml to get the most accurate number. Most of the time, my doser heads produce the same amount every time.
You do this for every head, then they are calibrated. It's nice to do it with the lines setup where you will have them. That way you know how the head will perform.
Also forgot to mention, make sure the lines are full completely before starting the calibration. This can be done with the manual dose buttons