Proofing your mss

Thw great opus is finished. You've run it through the spell and grammar checker and found those little errors. Have they all been found. Maybe yes and maybe no. They aren't going to find clam instead of calm or she instead of the and a number of other things. I often find this happens when I have two letters that are close together on the keyboard, like m and n. So can becomes cam. ran becomes tan, you get the picture. How to solve this since you want to send out the perfect manuscript for consideration? I've tried proofing on the computer but I've found that doesn't always work. Witness one mss I sent out and the wrong choice word was on the first page and the second paragraph. This is my tip.

I print out what I'm going to send out single space and then I sit with a ruler and read line by line with the ruler allowing me to only see one line at a time. This sounds like this might take a long time but I finished a nearly 300 page mss in about an hour. I'm not reading for story but for typos. I found one or two in each of the thirty three chapters. Did I find them all. I hope so but I'm not holding my breath. I've one book that has an error that I did not find, my first reader didn't find, the copy editor didn't find and neither did the editor.