Assuming your reference line is not down the center.
What does your composite look like?
You could fake it by splitting your center material in two. This will give you a live dimension point. For the line between use the white masking pen so it won't print.
Erika
Architect, Consultant
MacBook Pro Retina, 15-inch Yosemite 2.8 GHz Intel Core i7 16 GB 1600 MHz DDR3
Mac OSX 10.11.1
AC5-18
Onuma System
"Implementing Successful Building Information Modeling"