I have the Ansco Speedex Special R, which is the US version of the Agfa Isolette III.
My camera has the uncoupled rangefinder, but the markings on the rangefinder wheel are the same as those on the lens focus. So, when I am focused, I move the lens to the same point as the rangefinder reading. If the little dial for the rangefinder is partway between the 10 and the 15 foot mark, that is where I set my focus on the lens. I guess it may be possible to be off just a slight bit but like everyone has said, in most cases depth of field is your friend and any small focus errors are immaterial.
BTW, even at f4.5, which is wide open on my Apotar lens, my little Ansco takes pretty amazing images.
Here is a shot on Fuji 160 taken with the little Ansco. Would have looked better with a cow or three in the foreground but I couldn't find any at the time.