Put up an image of a film in the ratios you want to mask. I was lazy on my measurements on my latest masks and after covering them with velvet found that I was entirely unhappy with their size. The image would overlap the mask by 3/4 an inch at the top and bottom. It was really annoying so I painstakingly centered my pj to the screen, projected a 2.35:1 and a 2.40:1 movie on the screen and measured the bars. On my 120" screen I got 7.5 inches for each bar, the first set I cut to 8.25 inches. I am now very happy with the masks because on both the ratios I listed above there are no grey bars and no light over spray.