I don't know why this works, but it does. If you need a reference dimension that behaves like a driving dimension (i use this for "linked" dimensions) do the following:
1.) open a 2D sketch on a plane with orientation relevant to your needs
2.) make two parallel construction lines that are completely constrained (i.e. collinear with edges)
3.) sketch a point and constrain it coincident with one of the lines (call it "line A"

.
Make sure the point is coincident to the line and not to an edge or a point. You should be able to drag the point along the line.
4.) dimension from the opposite line "line B" to the point. DO NOT dimiension from line to line.
You should still be able to drag the point, but the dimension will give an error if it changes. You may need to correct an error when the external geometry changes. Do this by making the dimension driven and then making it driving again.
![[bat] [bat] [bat]](/data/assets/smilies/bat.gif)
On justice and on friendship, there is no price, but there are established credit limits.
![[bat] [bat] [bat]](/data/assets/smilies/bat.gif)