Thanks for the lead to these ruler thingys. They are really handy.
As far as my original question goes, I think the answer is related to pixels/inch, which in turn is related to the resolution of the screen.
I develop on a Mac with a screen of 1920 x 1200 resolution, or approximately 96 pixels/inch. When I scale the resolution on the display down to about 72 pixels/inch, an inch on the display is about inch on the physical ruler.
When developing a solution for deployment on another computer I guess we need to know the screen resolution of that computer and design the layout size accordingly.
You guys probably alreadty knew that, but I didn't!
Thanks for your response. It helped.