Yes that is basically correct. Personally (every set up is different) I have a vision enclosure that is larger (24x24x12) than some ppl use. The ambient temp of the location is 79º. This is the total ambient temp in the room and in the enclosure. Directly over the location of the UTH (it is on the outside NOT inside ;-) I have a hide. The temp of the substrate in the hide is about 90-91º the air temp inside the hide drops into 88-86º. The air over the hot hide is a bit lower again 85-2 and as I more the thermometer to from hot to cold about inch off the substrate I get 87º over the UTH 85º, 83º and at the glass from the coldest point I get 79-80. You need a thermal gradient. These numbers demonstrate a good gradient.
Hope this helps.
Alex