I don't know where that calculator came from, but it doesn't include a factor for reflector quality, so it can't be accurate. The total time the lights are on can vary from about 6 to 10 hours and still work ok. The longer the time, the more likely you are to run into algae problems. See http://www.plantedtank.net/forums/showthread.php?t=184368 for a better way to evaluate your lighting intensity.