Desk Top Dyno

I never played with a desktop dyno but can does it take the whole car and factor it or is that left up to the person after he gets a horsepower figure. Example if the car runs a radial or a slick.
They have a version that takes into account the whole car. We're just using the engine portion. But wallace power calculator says that it would take 720 hp to go 5.97 in the 8th. With a 3100 lb car. Assuming that everything else is working correctly. The wallace calculator is pretty accurate. It says that my Bloomer headed engine make 680 hp. It's been 5.85 at 115.6 and weighs 2930 with me in it.