It doesnt matter where the sensor is - I would get readings off the vine and the rock - when he is little he may sit on that vine but as he grows he will get to big for it - so get readings off of both of them and then watch to where he sits - he will probably use both -- but you want temps of 105-110 for a baby--- have you replaced the UVB bulb?As you can see the sensor is in middle but close to floor or does this matter??