I think what you are after are the paper based Octupus charts. From my basic understanding on what goes on "behind the scenes" these are prepared by a operators performance engineers using the Airbus Octupus software. They are airline, runway-intersection, config specific. With a take off weight, wind, QNH and temperature, you enter the paper charts and select a suitable flex. This is adjusted for things like low pressure and the use of engine/wing anti icing. The flex you select has to also fit within a whole host of values such as tref, tvmc. OAT etc. The V speeds for that particular flex and associated weight must be above the minimums and v2 must be equal to or greater than vmu for the given take off weight.
Thats a very summarised overview. Try googling Octupus and see if you can grab any off the internet.