I don't have too much trouble using 500 litres in a day - sometimes more if it's a busy one. I don't have an electronic flow controller though. I tend to have the flow full on at the tank and control the flow at the pole end. It really does depend what sort of work I'm doing though. If I'm doing maintenance cleans on glass that sheets well, obviously I will use a lot less water than on hydrophobic glass on first cleans. When the situation is right, I can manage on 300 in a day. I prefer to err on the side of caution though. I've never forgotten that when I first started with WFP, I lost work probably by not using sufficient water.
I know I have a couple of years or more under my belt with WFP but I'm not above spending a day out with someone to see if I can learn a bit more. Trouble is, I'm too busy to find the time at the moment. Would like to in a couple of weeks or so though.