I'm 5 weeks behind you at wfp (7 years trad) and probably am using the same amount of water per house! The way you clean was also highly recommended to me and that's how I was working today. It is pretty demoralising finding those spots and streaks but could happen for the first couple of cleans of my 150 customers so I have a tough couple of months ahead of me, hopefully followed by many years of clean windows and higher profits.
I have a ball cock in my tank so it can't get full - I haven't yet worked out how much capacity I'm effectively losing in my 500L tank, which is important at the moment, considering how much water I'm wasting!
When you're rinsing, do you always get a sheet of water flowing down for you to follow with your brush, or does it sometimes make lots of rivers and rinsing then takes ages? Panes right next to each, one could sheet, one could river, and even if I scrub and scrub the one that rivers thinking it's still dirty, it still does it. I'm thinking it's just one of those things, but rinsing takes a lot longer when the water rivers.