Good Afternoon, Luc,

As Clive notes above, the main purpose of a contact sheet is simply to let you know which negative(s) may be worth enlarging. Contact sheets are also helpful, however, for making notes about printing data in case you need to come back later for any additional prints from a given negative.

Being religiously consistent in making contact sheets can be an extremely useful practice. If you always have the enlarger set at exactly the same height, always use the same f-stop, always use the same exposure time, always use the same paper, etc., there will be a lot less need for numerous test strips when printing. A general rule for is to have the unexposed areas of the film get just enough enlarger exposure to be almost indiscernably identical to the paper's maximum black. On my monitor, your first sheet looks about right; your individual negatives look somewhat underexposed.

By the way, there is no need for a special contact printing device. Just get a sheet of window glass, preferably around 10 x 12 or 11 x 14 inches, to place on top of your negatives. Any glass shop should be able to grind the edges smooth; you may also want to super-glue a couple of handles near the edges.