It is important to keep the engineer's square, well… square in all directions. For the 30 series of 8020 profile and the Helios-Preisser 300mm square it is a bit challenging, since the edge of the 8mm slot is very near to the square's blade edge/height, and a slight mistake holding the blade causes the square to misregister. To help keep the blade at the correct height, I added this little screw/nut at the hole that exist at the tip of the square's blade. It supports the blade at a height that is very close to correct height, and it is rather easy to push the base of the square then flat to the base of whatever you are measuring. 

When modifying this to suit your square, try to keep the “nut” and the “screwhead” slightly short, so that you do not force the the blade too high.

And yea, they are not really screws or nuts, just interlocking knobs/studs :)