I'm going to ask the fitter to do this for me as I'm a bit uncertain how to go about it. Funnily enough I did ask him to do this during the installation before connecting up the boiler but he didn't have a flow cup on the van!
5l bucket or 1l bottle work well just time hoe long it takes to fill either and work it out how many you can fill in 60 seconds
for example if a 1l bottle takes 10 seconds to fill your running 6lpm etc if it takes 5 seconds it’s 12lm etc
you can do this via the filling loop will give an approximate gauge
eg turn the black lever off turn the blue lever off
disconnect the flex from the black lever side on the silver nut
put this into a 5l bucket or aim get your timer ready when ready open the blue lever fully and start timer etc