Basking temps of 84 F, to me, seem like the lower end of a good target to me. For exmaple, see the pdf below that shows some body temperature data for a few chameleon species that have similar habits to Veiled. Most of them, given the opportunity, are maintaining a mean body temperature in the mid to upper 80's F (low 30's C), but occasionally are maintaining body temps in the 90's and even up around 100 F.
Access to lower temperatures and the ability to thermoregulate is absolutely critical. If any species can't get away from the heat easily and cool down, they're in trouble, but I can't help feel that by providing basking area temps onlyy in the low 80's F, it could make it more difficult for the animals to maintain preferred temps. When they bask they can easily reach body temps higher than the ambient air temp in their basking area, but still, 80 F as a max air temp seems awfully low relative to where these animals come from.
When I used to have and breed Veiled chams, a number of years ago, I'd usually shoot for basking area temps in the low 80's for babies (though if when they basked, I'm sure their could get their body temps higher than that) and maybe 90 F for adults a temp of ~70-75 F in the cooler parts of the enclosure. That worked well for me. A key always is a temperature gradient. The chams need to be able to get warm when they want to, and cool down when we want to, and they only way they can do that easily is providing a slightly wider range than they'll generally want for their mean body temp.
Anyway, you can see a bit of data here:
http://www.naturalsciences.be/institute/associations/rbzs_website/pdf/abstracts_130_s1/130_s1_14.pdf
Chris
