i often used to clean domestic work in the dark in winter and never had a problem with bad workin fact i think most good window cleaners can squeegee a window with his eyes shut and still make a good job, i dont want to turn this into a WFP trad argument but today i did some high windows WFP and thought i had done a good job but when i went inside and the sun was on them they where pretty bad and i went and re cleaned them, with the wind we have had recently these windows where obviously worse than i thought and it was my error as i obviously hadnt rinsed enough but my point being is that working with WFP is very similar to working with trad in the dark in that you cant see the results of your work untill they have dried so you rely on your skill and experience to judge if you have done a good job.