I am having trouble with the drawing for a large assembly (over 52' long, hundreds of parts).

I have a number of parts that are spaced on 12" centers and the first one is offset 5.75" from the front plane of the assy.  I've created a section view to show the inside of the assy.

I would like to add an ordinate dimension (zero at the front plane) in the section view that shows the midpoint of each of these parts, so I have been selecting the midpoint of the line along the bottom surface of the part.  Everytime I do this, I get the window asking if I want the dimension driven or driving.  It also takes a really long time to accept either option (looks like it is rebuilding the model).  I also tried adding a centerline to the part and dimensioning to the cl, but that has the same mysterious rebuild time.

I thought that ordinate dimensions added manually to drawings were supposed to be reference dims and shouldn't affect the model.  Any suggestions of a speedy way to do this?