Surely when a large aircraft manufacturer works out premeditated and forecast performance they use more than just mathematical equations?
I am far from an expert on this so on my part its guesswork but they have access to wind tunnels where they can test and modify structures which make up the aircraft?
Take winglets which may be used on a progressive design?
they will be designed, tested and modified to create an effect?
They will be tested alone in the wind tunnel with sensitive instruments measuring everything from pressure to airflow and then tested with the wing?
That data will then be used to workout performance gains.
I am sure its not a case of sitting down with a blank sheet and saying x+y-z=127.7 KTS
Pace