During trial periods increase or decrease exposure times by a significant amount, eg. a 15 sec exposure might produce a negative looking underexposed. Do not make the classic error that many photographers have by increasing exposure times in small increments, with the frustrating waste of paper and time. As long as you can see the essential tonal detail in the negative, the document scanner will cope, but err on the lighter side as scanners, like our eyes are not adept at detecting tonal variations near the 'black' end of the spectrum. Watery' contact will have a small tonal effect which can be ignored. Inverting the scanned image and adjusting the tonal scale will reveal to you immediately whether you need to use more or less exposure.
Experience soon allows judgements to be made without recourse to wet scanning.
More work needs to be done, and it would be very interesting to see if the driver, from a quality scanner capable of scanning negatives, deals with the tonal aspect more faithfully. I think, though it's early days yet. The exposure times varied from about 15 seconds to two or three minutes, but I preferred to 'guess' the exposure by trials than try to adapt an enlarging meter, but now I have satisfied myself of the success of using paper negatives, an adapted enlarger exposure meter might prove invaluable in future. Being able to see the image, albeit a negative one come to life in the developer dish in a few seconds after exposure was a welcome change to normal film processing, and although the exposure times are lengthy in comparison to film, overall an image can be displayed on the PC screen in a much shorter time. Low Tech' operation requiring the minimum of photographic skills and equipment for DIY processing.