choosing your measuring device
Probably a yardstick is not the best measuring instrument to use for this.
For that matter, neither is a footstick either (the 12 inch measuring tool in which most North American youth have experience with) since it is one inch too short for this GUI which was probably designed by some European UI Expert no one ever heard of.
The first time I tried to do this, all I had were a handful of 800mm rules that were made of metal and I did not like using them on the glass on my computer. I would like using them even less on the surface of my LCD Display.
A flexible sewing tape measure is probably the nicest on your equipment. Your measurement will be more accurate if you use millimeters.
The little piece of 35mm film is a really good thing to use for this if you have it. The piece of film will probably not hurt the monitor nearly as much as your fingers are hurting the film as they hold it. Almost every film developing service I used was religous about returning the entire original film, including the little ends that are used to load the film into the shaft thing. These little unphotographed end pieces are perfect for calibrating your monitor with, if you can find one.
Inspite of my complaining about the European design, the dialog is a measurement of the porportions so using just a 35mm measuring area should work in the dialog. Although, I haven't tried it to verify.