Wattage only determines how far away the lightbulb should be.
A 100W bulb 2 feet away from the cage isn't going to be hot enough for an adult cham, but even a 40W bulb inside the cage that the cham can climb onto will burn the chameleon.
So what you actually need to do is measure the temperatures at the basking spot (and at the spot where the cham can get closest to the bulb), and then adjust the distance of the bulb accordingly so that the temps. are within the recommended range. (Of course, you also don't want the bulb too close to the cage, so if the bulb isn't powerful enough to provide those temps from about 6 inches away from the cage, then you can get a higher wattage bulb, but I'm sure that won't be necessary with a 100W bulb).
For a veiled about that age, you'll probably want a basking spot in the mid to high 80s.