If you use the correct resolution for your monitor, and you want to view an image at an aproximate dimension in inches, you can use the 72 DPI rule, you are not that far apart. Correct resolutions are 800600 for a 15", 1024768 for a 17", 12801024 for a 19". Then there are blockes like me that uses 1152864 in a 17"....obviously, I must change the numbers if I want to see how big would be my image when printed. I mean, that is only a basic rule, and EVERY photo manipulating program uses that as basic setting. And yes, it's almost meaningless, but it's a basis :)