hmm. there might be a problem with your explanation there might not be. But I'll try to give some experience and some advice anyways.
If your trying to get true metric/imperial sizes for screen size. Don't even try. I tried this myself. Supposedly browser/screen makers should be giving out accurate dpi for programs to offer a way to get "true" sizes. They don't. You can create a program or anything that has say 5 centimeters in measurement. You will never get that 5 centimeters. I went around 4 different size monitors, 4 different sized mobile devices(10", 9", 7", 3.5") and my 1 inch comparison never really matched up to the ruler. Larger monitors were far closer though. This is not a issue with C2 as it is with browser makers or possible screen manufacturers.