Looks like 24 at the base, 18 at the top. I might try to make it 24 all round though, but I can understand wanting the slant. Another thing you might do is raise it off the floor a bit; I've heard Chams get skittish closer to the ground. I think one important thing is adding some more screen, maybe a couple panels to the side and the back.
Thats just the cage part i have a stand ready. The cage does slant giveing it a cool look, and to keep the humidity up for a montane species i am going to keep the sides and back solid. The floor of the cage is going to be slanted to the back and a 1/4 inch strip will be cut allong the back to let water drain into a gutter to a bucket.