CCF above air mattress if it's uninsulated, CCF below if the air mattress is insulated. If you put the CCF under an uninsulated air mattress you run the risk having cold air come up though it.
I use an insulated air mattress in winter and lighter, uninsulated one in warm temps. Since I have insulation in the air pad, to me it is as much about protecting the air mattress as r value of the closed cell pad or added comfort. I also use the closed cell foam for a sit pad, lounging in camp, shooting mat, gear pad, etc. If you are counting ounces seek outside makes a very thin mat that has a bit of r value and protects the air mattress.
I've used a Nemo Switchback cut in half underneath a Thermarest NeoAir Xtherm for over 120 nights. Both are great pads together or separate. The Switchback is a great closed cell foam pad. It's just a bit thicker than the Thermarest Z-Lite.