Something that I think could make this tool really useful, particularly with high-res LCDs and laptop screens, would be the ability to switch on 2 magnified areas at the start and end lines for the measurements.

The only other time I've seen software do this is in Corel Capture -- it has the option to "show area in zoom" when taking an "area capture" from the screen. It provides a small square window (no frame) that you can move any where on your screen showing the magnification, and a set of cross-hairs that represents the centre point for the magnification.

Ruler tool could do a similar thing, but instead just place the magnified areas at the top or bottom of the ruler and have them dynamically follow the start and end measurement lines as they are dragged.

Adding this kind of feature I think would allow for greater accuracy and certainty when trying to measure from higher-res screens.

I have attached a small screenshot of Corel Capture's zoom feature in action.