You have to add sketch lines to the drawing and dimension to them. Add a line with a vertical constraint to the left side of the view, and give it a tangent constriant. Do the same on the right side, then dimension between them. You can put the lines on the dimensions layer, and tune the lengths by hand to look the way you want them too.
You might be able to move them to a new layer and turn the layer off if you're really picky, but I've never tried it that way.
Ben