GB , flow rates dont mean much , ive been through all your doing at the moment, i used to spend ages trying to cut the flow rates down , some time later i took a step back and thought about it.
If i cut the flow rate back it would take me longer to clean each pane , chasing the dirt down with a dribble is not the way to go , this is when you get poor results , what you need to do is bang up the flow and not worry about it , use more water and do it quicker with better end results , ok your gonna use more water but you will earn more in a shorter time doing so , thus justifying the cost of a higher flow rate .
I could go out in the morning and alter my flow on every house and still get top results using 1lpm to 2lpm , how ? , because i know the characteristics of all glass on my round , it comes with time .
And to add to Count Phils comment , about getting your flow rate , putting your hose end into a bucket is not right , what you have to remember is all setups are different , me for instance , 6mm i.d microbore , then goes to 5mm i.d pole hose , then to fan jets .
It all boils down to what you have at the brush head , 2mm pencil jets will release more than fan jets at the same pressure / flowrate.
Time a minute with your brush head into your w/c bucket , nab you birds measuring jug and bobs your uncle.
Hope this helps
Rich