We were in the Culbin Forest near Nairn on the Moray coast of Scotland. We climbed the tall viewing tower on Hill 99. You could see Morven, a prominent hill that is a long way up towards Wick. The panel board tells you its height, which works out at about 700m. If my finger blocks off 1 degree of view, I'd estimate Morven at 0.5 degrees. That works out at about 0.009 radians. The distance to Morven is then 700/0.009 = 80000m or 80km. I measured it on the map as about 70km so the method isn't bad. It's not quite 50 miles.
