mk11,
I found my quickest work thru was to place a line from the exterior centerpoint of the opening that was above the floor plane down (blue) to the floor, this was my pick point for dimensioning. Mostly I deleted them at the next ISO view, but if you place them in their own layer, you can use them again.
I like the dimensioning capabilities of SU. It is extremely flexible,stable and easy to modify, and with a little technique modification on my part it is quite easy to dimension a plan to my tolerances.
Regards,