I have found that it's not that critical, Mike. I also use my hand to find all the high and low spots. However, you can leave a high spot and you won't be able to see it, provided the high spot has no sharp edges to it. I've left some errors on the rentals that I would swear would be noticeable, but, after the paint goes on, it looks just perfect from two feet away. I suppose that you could find the error with a strong light and a distance of six inches, but, for the inside corners (the usual problems) at the ceiling, why bother?