I bought one of these, and have been using it over the past few nights. I have a Durst M601, and am printing on kentmere luster VC paper. I setup the aperture of the lens at F8, and print for 60s. After developing and fixing, looking at the print I select the tone that I like from the scale, and develop another sheet. What I've found, however, is that after developing the next sheet the selected tone from the scale, and the tone that I get on the print at the same location are not the same! Generally the tone looks to be be about 1 stop underexposed (ie: if I selected 16 seconds, the tone looks more like 8 seconds). What could be the reasons?