Here is an easy navigation trick to quickly estimate straight line distance between points on a map. This works for maps at any scale, provided you have a scale bar, which most maps should have.
Take a twig and break it to match 1 mile or 1 km from the scale bar on your map. Or maybe 5 or 10 miles, if you’re using a very small scale map. Often the mile and km scale bars look very similar and are right next to each other. Be sure you’re using the correct one.
Now, just leapfrog your “1 mile” twig across the map to get a quick estimation of straight line distance between real world points. To measure a curving distance like a trail, road or river, it’s better to use the “finger” scale, learn how to do that here.