To Dimension a Distance Perpendicularly from an Element (at a Separately-Identified Point)
Select the Element Dimensioning tool (
Enter a data point to select the element from which a distance will be dimensioned.
The orientation of the y-axis of the dimension is perpendicular to the line on which the element lies.
Set the mode to Dimension Size Perpendicular to Line.
Enter a second data point to define the origin (the base of the first extension line).
This point can lie beyond the end of the element identified in step 2.
Enter a data point to define the base of the second extension line.
The dimension is placed with extension lines at each end.