It wouldn't matter how the tall the cage is...it is how close is your basking vine, branch, etc beneath the light. Does it provide enough room for his casque not to touch the screen where the light shines down and he is basking? I mean you could have a 10 foot tall cage and it you have a branch sitting 6 inches below the light, and their body can touch the top, then they are gonna get burned.
yes, and you have to take into consideration they will grow if you get a younger one. I mean what worked safely 3 months ago may not now. I almost made kinda the same mistake and one day I looked and my chameleon was puffed up under the light and his back spikes were almost touching the screen. I had never realized this when he was not puffed up.