Like I stated in your other thread, you can start with a 60w bulb and measure the temp on the branch. If its too cold, use a higher watt bulb like a 75w.
If you find a bulb that gets you within a few degrees of the target temp, you can adjust the branch height either high to increase the temp or lower to decrease. There is no exact science to it since no two light bulbs put out the exact same amount of heat and light.