Select the stand(s) you would like to calculate the open sides for then click the 'Calculate Open Sides' button which is located on the Dynamic ribbon.
TIP: You can also initiate this function by using the default Hot Key ' O '.
NOTE: The Dynamic ribbon will not appear if you have entities other than stands selected, to deselect the other entities and only select stands, press F3 twice.
Your selected stands will now be calculated and show the outer lines of stands as dashed lines. If some of the stand edges need to be changed manually, use the Toggle Open/Closed Sides function.