It is difficult to tell someone what the best wattage bulb for their enclosure is because every enclosure is different and other things play a factor like room temp and drafts.
Your best bet is to get a thermometer with a probe or a temperature gun and take multiple readings over a couple of days. Put the thermometer in the hottest spot your chameleon can get to.
For a 2 month old panther I wouldn't make the basking spot over 85 degrees. Be sure that the top of your chameleon's head and his/her back can not get any closer than 6 inches from the bulb or it could suffer some nasty burns that will leave permanent scars. Taking these into consideration, you can raise and lower the bulb to lower and raise the temperature as needed. If your bulb is 6 inches away from the highest point on your chameleon and you still can't get the temperature high enough, then it is time to try a higher wattage bulb