Actually you need to fudge the adjustment so it's closer in the mid range and slightly out a 3ft. It's a balance and while I agree with Peltigera that it's rasy to guess but for more ctritical work they are useful.

I have a few rangefinders, some remarkably accurate others require new mirrors (which I need to cut) and alignment, most cost around 1. I just paid 99p for a Kodak close focus rangefinder which comes with it's own close up lenses.