Symmetrical Dimensioning
2-D Dimensioning + Text > Edit > Extras 
> Symmetrical
dimensioning 
If you select a symmetry line, you can identify the line to be dimensioned by means of just one point. The symmetry line must lie at a right angle to the line.
- Identify the symmetry line and then the start point of a line.
- Specify the point for the dimension figure.
You can now choose the start point of another line or right-click the function to end it.
(1) Symmetry line
(2) Start point of the dimension line segment
(3) Point for dimension figure
(4) Succeeding point
