Are there any engineers out there?What is the formula to find how much heat is generated by a parabolic mirror

There must be a general formula for figuring out the amount of heat at or close to the focal point of a parabolic mirror. I have seen demonstrations where a two foot mirrror can catch a piece of wood on fire so I know it must be turning out at least 400+ degrees and it is was obviously a warm day in the video. I would be more interested in what a 6ft. - 8ft. mirror would generate in a round area of about 6 inches, which as I realize would not be at the maximum since it is not at the exact focal point but slightly ahead of it, and towards the mirror. Any formulas I can use? For example if it is 80 degrees F. out about how much heat could a 6-8foot mirror generate? Thanks for any help on this problem.