Although I haven't tried this yet, I wonder if using a trigger might not be a far more effective way of saving water.
The idea is to wet the window and then turn the water off and give a little scrub, soak, scrub, soak, scrub as much as may be needed and then rinse.
The idea being to allow both the purity of the water and the agitation of the brush both do their fair share of the work. I'm convinced that probably most of the torrents of water I use isn't doing very much good. Maybe an occasional trickle of water and a scrub would be more effective at washing and then just a quick torrent for rinsing.
I've certainly used this technique on tenacious bird poo. I won't leave 2 litres of water per minute flowing while I'm scrubbing a tiny bit of baked-on mucus. It's a quick blast, then a scrub, then a blast and scrub until it's gone.
Also do the same on the bottoms of frames where there's green alge and muck trapped on the little horizontal line where the beading meets the frame and in the sharp lower corners. This needs agitation more than it needs water. Makes sense to me to turn off the water whilst doing the scrubbing.