I am building a 2D dynamic block and having an issue. I have a top view of an open door. The goal is to have the Panel Width scale in size as the Door Width changes. see below: 
I have tried to control a polar stretch from a linear parameter. Not good. I have also tried making the "Panel Width" expression a formula using "Door Width" as a variable. I get a syntax error. I have been able to use user parameters in formulas before, but not in Action Parameters. Does anyone have another solution or a suggestion?

I am using Auto CAD 2014

I haven't experimented with this, but my first thought is to use a parametric constraint.  Let's say your total width will be divided with three panels.  Therefore a panel length is equal to w/3.  Play with this idea if you haven't already.Dave.