I make a "test strip" using a 8x10 sheet of paper. The first strip is 2-3 seconds less than I think the exposure should be, the second is what I expect the correct exposure to be based on previous test and past experience, and the third is 2-3 second more than I expect the correct exposure to be. This way I see the whole 8x10 print, I see the correct exposure (hopefully) and I see what areas of the print would look like at more and less time than what I think the correct exposure would be. I've never been able to evaluate a full print from a small section (test strip).