I use Screen measurement pack. Used it for years. I would ask for the 3.2 version of Screen Calipers, as the new one does not allow you to set to screen measurement.
Let's say for instance, I have a screen of a topo map showing. I go to whatever resolution I choose, and then go to the bottom and set the screen calipers for the distance from 0 to 1 mile. Next, I can go to any reference point on the topo map and measure to wherever I want to. Now, a caveat to that, is that IF you change the zoom level for the topo, you will need to go back and repeat the calibration.
Now, I use Screen Potractor sometimes from the same company, to determine the angle of point b from point a. I can then assume that the angle will be consistent on Google Earth.
So, I get the topo or other reference map calibrated, then switch to Google Earth. At this point, I use G E's distance measurement, and sometimes, use the Screen Protractor to show me the relative angle from whatever point A reference to Point B destination is.
It is much easier to do this, than to explain it.
I have used this for years and it works. I also use Overlays as well. I will tell you in my part of the country, the roads have changed significantly, and the topos are not always right on. My method is, imho, faster in those instances.
My $0.02 worth.
HH
Gitterdug