The idea is that the Uzo can be used to estimate the range by comparing the target's mast heigh to the marks visible on the Uzo. I haven't used Olc's Gui but from your description the Uzo seems to be similar to MaGui F. One can make a chart like the one shown in one of the screenshots here:
http://www.subsim.com/radioroom/show...8&postcount=65
Olc's Documents should have info on the marks chosen for the Uzo. Different GUI's seem to have different values.