I wish to create a sketch composed of arcs and lines that has built in relationships that will cause it to behave like a simplified version of the cross section of a flexible diaphragm.  For example.

The way that I did this in the above sketch is to define the left hand vertical leg as a fixed dimension, and then make the right hand vertical leg length a quanity minus the left leg length (30 in this example).  I have tried several methods, and although I can make this work in a static situation, it is quite clunky.  What I really want to be able to do is to make this work inherently, like the fluidity of a tangent relationship.  Ideally I would simply be able to make the length of the combined line equal to a constant. 

Any ideas?