Making dimensions oblique    

Extension lines are normally created at a perpendicular angle to the dimension line. You can change the angle of the extension lines, however, so that they tilt relative to the dimension line.    

To make oblique extension lines

   1   Do one of the following to choose Make Oblique ():     

    On the ribbon, choose Annotate > Make Oblique (in Dimensions).

    On the menu, choose Dimensions > Make Oblique.

    On the Dimensioning toolbar, click the Make Oblique tool.

    Type dimedit, press Enter, and then in the prompt box, choose Oblique Lines.    

   2   Select the linear dimension, and then press Enter.

   3   Type the obliquing angle, and then press Enter.      

You can align the oblique angle if you dont know the exact measurement.
Use entity snaps to pick two points on the entity.