If you use a screen with the grid on it, the first lines from the top and bottom or left and right show you the 645 image area. See parts 3042217 or 3042260 on this page; http://www.hasselbladusa.com/product...cessories.aspx

I've heard of people drawing lines on the mat side with a sharp pencil, but I prefer the grid screens anyway, so for me, that's the easy solution.