Minimum line length definition

Hello,

I'm trying to build a parametric section for a retaining wall.

I have two parallel lines and I want to define a constraint between these lines so that line A will have a minimum length of line B. 

   

Currently I tried doing so by using the minimum dialog box as you can see in the screenshots above. But when doing so, (I define the minimum value of line A as the length of line B) and after validating that the expression is ok - the model doesn't change. 

Is there a solution to this problem?

Thanks in advance

Alon Kopilenko