Not sure how I keep something in the middle while stretching/changing size of geometries around it

[nilswa]

I want to create a parametric constraint that spaces a wall evenly/ in the middle of a geometry/room. Every time the geometry is resized on either side it should still be kept in the middle. How do I do that?

[j.palmeL29YX]

First you need some additional constraints to full constrain the geometry. 

Then you can either set d2=d1  (or d1=d2) 

or you add a coincident constraint endpoint of the vertical line to the midpoint of the horizontal line. 

>>demo<<

BTW: This issue has nothing to do with dynamic blocks (until now. We don't know what you further want to do)