AutoCad :: Using Constraints To Stretch Multi Lined Arc

I have created a couple of dynamic blocks for profiled window surrounds for both a square top window and arched window. I have accomplished the square top window, but creating the arch top has proven quite difficult.

What I need is roughly the same setup as the square top, using the XY parameter set and entering the width and height so the window stretches accordingly, however on the arch top, I am needing to add a parameter to enter the "chord" dimension. I basically need another user imput parameter to facilitate the peak of the inside radius (currently shown at 4'0-5/8") in the attached drawing bordered by "DYNAMIC BLOCKS." I am not sure on which parameter to add to stretch the arch at the designated "arrow." and even once that is figured, how to make the arches that are stacked above it to stretch properly. I added Horizontal Dim constraints which seemed to work, however upon further review once stretched, the outer arch doesnt stretch properly (Review Example 1). It is supposed to be like "2" and "3". Basically as you offset the inside pline out 6" the intersection is supposed to adjust and not stay stationary.

Also on a side note, I added vertical constraints and Dim Vertical Constraints to the "legs" of the window profile because as the inside arch was stretched the legs would not produce logical results.

WINDOW EXAMPLE.dwg