In a drawing, I am attemptiong to dimension between rectangular or square tubing created as structural members.

When adding dimensions to a drawing between lines, I generally hit the Smart Dimension button, pick the first line, and then pick the other line.  With tubing specifically, picking the second line usually places the dimension from the first line rather than across the two lines.  It seems I have to be incredibly accurate with the second click to dimension between the two lines.

Is there a reason tubing is more difficult to dimension than flat edges?  Does it have something to do with the rounded edges on tubing?

How can I make dimensioning these types of parts easier?