With the ambient temps that warm probably a 75w should do it. You should be using a digital temp gauge with an external probe to check the basking temps which for starters can be around 95F but what you really should so is observe to see how long it basks for … if it for a short while, say for just a few minutes then the temps are probably too warm but if it bask for a long time then you may need a higher wattage bulb. So you will need to experiment.
I can really answer that. It depends on the room temps, the size of the chameleon, basking light wattage. Just remember that really long periods of basking … and I will guess like 45 minutes to an hour (someone please correct me if I’m wrong) is probably too long and the wattage needs to be increased or the distance from the bulb to the chameleon needs to decrease. Just don’t get the bulb really close to the chameleon … you are risking burns that way. I always go with a higher wattage bulb rather then to put the basking light right on the screen.
If the chameleon basks for very short periods, maybe just a few minutes or its skin turns white and/or it gapes with its mouth then the temps are much too warm and the distance from the bulb from the chameleon needs to increase or use a lower wattage bulb. I can’t give you definitive answers really, as I said you need to experiment and be observant.