Without ruling out
I must confess that I sometimes find myself putting a ruler over the monitor to check distances. Mostly because I am feeling to lazy to take a screen shot and copy it in an image processing tool. If you're like me, then you might be interested in MB Ruler.
MB Ruler is a measuring tool box and can be used to measure and track elements on your computer screen. It features a triangular ruler that helps you detect angles, a coordinate finder with rotating coordinate bars, a distance measure tool and a system for marking points and coordinates. There is also a collection of customizable tools that vary from simple lines to the wonderful, flower of life. I highly doubt that MB put it there for its usefulness in measuring screen elements.
I like MB Ruler. Although I'll seldom use it, I think it's quite versatile in its design. I only wish it would not rely on right clicking so much.