Hi DS fan,

 How do you draw the perpendicular line to the center of an inclined line ?

MY SOLUTION : :

Start by the StartPoint external to the reference line,
   L and so on

RMB : ESNap override / perpendicular
connect to the indicated point ( a 90° angle symbol)

Then move to the pertinent point.

Thanks to Robb and a good night.

May be another way ?