You want to derate thrust as much as possible to save stress on the engines. And you can use TOPCAT to act like your dispatcher which will give you your derated thrust rating, that is it, which takes into account the lenght of the runway and how easy it will be for your aircraft to get up to V1 before you are too far down the runway.
The 744 has the speed charts stored in the FMC, although you will have paper copies in your flightbag. You want to fly like a real pilot? Real pilots do not calculate takeoff conditions, that is left up to the dispatcher. If you never do the calculations your self, how will you know what values to chose and why? You must chose the flaps setting and the engine Flex temp. 1) For the situation I am in, and the settings I have chosen, how fast can I go before I can no longer stop? 2) What speed will I take off at?Why do the calculations yourself when the FMC can do them for you? Because you must chose factors that affect them. The purpose of all those calculations is to give you two very simple bits of information. Once you reach V1 you have no choice but to take off. That speed which changes the decision is V1. More power means shorter roll but gives less time to deal with stuff and increases engine wear and fuel usage but there is not much use in saving fuel if you run out of runway before you take off! If something goes wrong during your take off roll, you need to know if you can stop safely on the remaining runway, or if you need to takeoff and deal with the problem in the air. How much runway do you need to reach that speed? That is dependent on engine power, which is controlled by altitude, pressure and temperature (real and flex). Feed in the factors, and the FMC (or the charts) will tell you how fast you you have to go to take off.
Vr, the speed you rotate at, is dependent on your takeoff weight, flaps setting, and head wind.
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