When I did mine, I found a web page that described the geometry. There are two, perhaps three variables. First, the distance from the center of rotation of the butterfly shaft to the cable attachment. Second, the angle of the total cable pull for the TV cable.....the angle is a function of the distance so both must be right. Third, the relative position of the adjustment block relative to the butterfly shaft.

here is the way one guy did it:
http://purplesagetradingpost.com/sum...o/700R4p7.html

mike in tucson