Also a very old topic, but I am forced to post to this, since there´s not been any answers to it yet.
I set up a grid of construction lines around my floorplan, and then use those nifty little automatic temp construction lines to guide my placement of the dimensioning lines. In addition I create a second set of clines that describes the length of my extension lines.
I just now noticed that you can create an offset for the extension lines, so that the actual measuring point and the start of the dimension extension line are apart.
But I wouldn´t mind having a "set length of dimension extension line"-setting, too
EDIT: A few other threads on this and similar topics:
German and Australian extension line style request: http://forums.doublecad.com/index.php?topic=759.msg3457#msg3457
Offset to zero not possible by design: http://forums.doublecad.com/index.php?topic=735.0
Unanswered questions about Dimension style palette: http://forums.doublecad.com/index.php?topic=735.0