Hi m.b.s
The problem will be the jets on your brush. They are most probably 3 mm jets which use at least 50-60% more water per minute than 2 mm jets which is why we always fit 2 mm jets as standard to all our brushes.
As to getting a good result, you will need several weeks of practice before you can guarantee your quality. Until then, you will have to check every window several times to ensure a good result. After a couple of months you will be twice as quick and have a quarter of the problems but it is not as easy as it looks to start with. Don't worry, you are only experiencing what everyone else experienced at first. Keep on cleaning your own windows and friends and relatives, and it will get easier.