The sag should be about 100 mm. A lower sag makes the back end stiff and will bias the weight to the front end and over load the forks, Too much sag will bias the weight to the rear and the front can become unstable and push in turns. In either case the sag will affect your suspension dramatically and there won’t be enough adjustment for rebound and compression.
I assume the “nut thingy” you refer to is the large nut that controls preload on the spring. In your case you don’t have enough sag so you need to loosen the nut (back it off counterclockwise) until the sag is at 100 mm (check a manual for the correct sag value). 2 turns of the nut is approx .300” of sag. There are actually 2 nuts that lock against each other once the sag is set.