I used to do a job at 80 feet in & out.
Out first day, inside the staircase (full height) second day.
Hand on heart-after the first clean there wasn't a mark on the glass, not even the odd drip.
An element if guesswork-perhaps. Did the job for 5 years odd. Perfect every time.
Don't do it anymore as it wasn't my job, it was subbed. The guy who had it still does it, far as I know.