The center height sensor should be on the right end of the head. There are two of them there.  It is just a small black potentiometer.  The only one for contour master should be under the feeder house I think.  I have a RAHCO leveler but it uses contour master sensors and that is where it is located.  I would think the head would have to be unlocked for the head cal on the center sensor. Never lock the head here so not sure.