Built into all serious computers is a line of code that calculates "sigma" the relative air density. If you want to do "sigma" manually try
http://wahiduddin.net/calc/calc_da_rh.htm for very good on-line calculators. Next you need a glance at TOSTA, the takeoff safety training aid provided by Airbus Flight Operations Support & Line Assistance. (Issue 2 -11/2001?) Inside the computer also will be look-up tables of effective thrust plotted against ram drag and lift coefficient plotted against V LOF. The first guess of flex will be based on (Square of Weight)/(sigma x wing area x lift coefft x effective thrust).Then the computer will check its legality within certain bounds. The Boeing does the same, it's physics after all. The better airline POH's require you to look at Rel H or dew point, the less well thought out ones ignore it. Ah well!