Lightware SF11c ground reading too high

I have Pixhawk 2 and have set the Lightware SF11c RNGFND_GNDCLEAR to 31cm since that is the how far it is off the ground. (Large Copter) I get a 0.66 altitude reading when on the ground. I would expect 0.00 It is connected to GPS 2 (Serial 4) Any thoughts on what I’m missing? I have all the setting per Arducopter documentation and it is working.