It would be interesting to see if you do this by reference to know points
No, I don't do that. I simply estimate an inch. I used to do it on OS maps, where it represented a mile, for years and years. Now I do the same thing on half mil charts, but it's 10 miles. I simply count, using the tip of my finger along a line, or even without a line. I'm always pretty close.
As formationfoto says, surely most of us are capable of doing this. And personally I think it's far easier for in-cockpit diversions or similar than messing about with measuring devices such as pens or thumbs. Also for rough estimates...shall we fly there tomorrow? Roughly how long will it take? Let's see, I make it about 110 - 120 miles...that sort of thing.
I really can't believe that no-one else does this!