Hi Gary,
I do tops and bottoms with a 300 L tank and I always come home with 50L at least now, I use an inline tap on my pole wet the window, turn off the flow. scrub the window, turn on flow and rinse, turn off flow, move to next window, since switching to microbore and fan jets I use even less water than before. My best so far is 22 houses, 6 are 3 storey, the rest are a mix of 3 and 4 beds. all UPVC never had a complaint since using this method.
I always keep the flow going for 1st cleans and yes I wish I had bought a bigger van so I could have gone for 650L system.
hope this helps
Greg