cam_tilt value is in degrees of the total range of camera movement, so a value of 0 would be (I think) fully tilted upwards, and a value of 180 would be fully tilted downwards.
You'll need to start with 90 and add/subtract as necessary.
Looking back over my code it looks like I've confused things by saying:
# Default Pan/Tilt for the camera in degrees.
# Camera range is from -90 to 90
cam_pan = 90
cam_tilt = 60
And then doing this:
# Turn the camera to the default position
So that a
cam_tilt value of
0 would result in
-90 being sent to the PanTilt HAT.