I only gained confidence after a year and a half of cleaning off the glass.
In that time (apart from 1st cleans) i would turn up for a job and just start cleaning asap so i only ever saw a wet window. And therefore couldn't really tell how clean they still were.
Eventually i started to use a higher flow rate and then walked around a few jobs (when the customer wasn't in, so as not to give the game away

) and checked the windows. Some sides were dirty but others clean. On the clean windows I then started just to "throw water at the window" and move on to the next.
The following month i checked again and they were good.
I then started rinse on the glass.
I still have jobs and windows that i have to spend more time on and rinse individually off the glass but they are fewer and fewer now as i get more adept at sizing up what needs to be done.
One effect it has had though in the last 6 months i have had many customers accuse/tell me that i am too quick. I can see myself losing a job here and there because of resentment at what i charge for how long i take now.
This pees me off but it won't change how i work now. I will only slow down when i hit a problem or get to the stage where i am that quick, the job hasn't been done properly.
I guarantee my work to cover this scenario, so am not too worried about the future.