Boiling point is subjective and will introduce variation in results, especially if you don't adjust for barometric pressure. A better method would be to measure how long it takes to attain a temperature rise, say from 50F to 200F. Then you can calculate stove output in whatever units you wish. You can also calculate the efficiency of your setup by weighing how much fuel gets used in the process.

I suspect boiling point gets used because the average person doesn't have an appropriate thermometer. The problem is that it takes quite a bit of energy to go from the onset of boil to a full boil, so deciding just when to stop timing will significantly affect the result.