margin measurements.

[manvbf]

I am trying to set some margins and finding that it is a hit and miss proposition. So I am thinking that maybe I can make some kind of visual representation that I can hold up to the screen to get a general idea of the margins in pixels. Is there some kind of rule like 800px = say 5 inches? I realize that there is all kinds of variables and it's probably impossible but I thought I would ask anyway