Unfortunately there is no marker of distance.
I just try different positions following a dichotomic strategy (start by the extrems and the try the middle of the interval each time).
I have noticed that some raw files have an exif info concerning the focus distance. So I don't know why this is not used automatically at least for raw files that contain this information.
Thanks. Its unfortunate. The distance scale feature in DLO seems to be incomplete. There is very little on how to use it and without markers, you can even accurately replicate the distance from shot to shot. Also depending on the photo, the changes in the distance scale seem subtle at best unless you compare at the extremes. Well, its free, so I guess I can't complain too much.
And with DLO applied the results, again depending on the shot, ranges from marginally better to spectacularly better. When viewed at 100% many shots before applying DLO look almost blurry by comparison. The effect reminds me a bit of the Clarity control in Lightroom 4, except the DLO results are significantly sharper with a lot more fine detail (line the rough surface of rocks) which seems to magically appear.
!

