How do I capture a Driven dimension in DriveWorks Express?

I have a right triangle in the top left corner of the frame I show on the right.  I have a fixed dimension of b=1.3"

The 2 angles A & B change and other two sides a & c change depending on the input of the sides of larger frame, H1, H2 and W.

SolidWorks gives me the results of the changes in the large frame but how do I grab those results to use in my formula?

I need to subtract the Driven dimension a (.7506) from the hypotenuse of the large frame.

a=(.7506)

(SQRT(((WindowOpeningHeightH-WindowOpeningShortSideB) ^2)+(WindowOpeningWidthW ^2))-.5) -a