ahhhhhhh, look on page 3 of the ds18s20 spec sheet.
0 celcius = 0000 0000 0000 0000
-0.5 celcius = 1111 1111 1111 1111
see, there is no formula for temperature in between these two, the calculation is done only ( by default with COUNT PER COUNT and COUNT REMAIN. which is obviously not good, not sure yet how I am going to fix that, working on it...