No, no, Rainboe, you can learn a lot from knobbers like this. First of all you can learn to persist in illogical arguments, despite the fact that your question has been answered by professionals. You might think those exams you took and the every-day use of Perf A calcs are important but, as you can see you're wrong. So, from Mr K.Nob you can learn how an aircraft REALLY gets airborne by counting the number of seconds (whilst seated in row down-the-back) and comparing it to how it usually feels. He's done this many times, so what would the operating crew know. What's even better, is you can also learn all about auto-brake settings. Not from the landing weight, vacating distance, turnround time versus brake temps etc but, no; now you can learn using Google Earth.
On second thought, you're right. S&T you are a tw*t. You've got your answer now piss off to some Microsoft forum and discuss how real professionals do it.