"Mean Curve" Or "Contour between PlanarSrf"

Hello,

I'm new to grasshopper and trying to work through tutorial definitions. I have done one from formularch.blogspotMean%20Curve.zip that allows one to make a "blended surface" between two  non-planar surfaces. I have tried to amend it to allow me to make a "blended surface" between two planar surfaces to no avail - basically the Rhino command "mean curve".

I will attach the original definition along with the way I have tried to amend it.

I would greatly appreciate ideas about how to proceed...

Thank you,

CML