Did you know you can control the water flow on the aqua-dapter for downstairs windows-when theres lots of windows(conservatories etc) just by turning the tap by hand to the desired flow.
But in my imo your best having a set rate, if its to slow your cleaning the window longer, or higher & clean the window quicker, get the flow rate right & consumption will be practically the same, you only need to save water when your not cleaning with it, or when there's a stubborn stain(bird muck, snail trails) turn tap off, scrub mark, turn tap back on.