donut king,
We do take lots of care in making the Cat A takeoff profiles, and the FAA/CAA do lots to check them too.
We determine the performance by trial and error, based on a core procedure that we have simulated. We vary the acceleration, cdp altitude and speed until we get something that works well, is repeatable, and doesn't need excessive skill. Then we test that procedure under varying weights and power levels, and also at the various altitudes that the aircraft is approved for. The testing is fairly extensive, it usually takes several weeks of flight test to get a workable procedure, then a few flights at each check point, and at each extra altitude.
The flight manual areas that describe the operating limits, normal and emergency procedures are all legal documents, and are written, tested and changed with great care.