(BA) We used to use paper Perf Manuals that basically used 1+F everywhere, except when field limited e.g. ABZ... then F3 used.
We now use some electronic gizmo called CARD via ACARS. This uses a random number generator for Flaps and Speeds, seemingly totally unrelated to weather or runway length. It is quite happy using speeds ~120K and F3 from LHR on a nice day (A319 - and include a 5K V1/R split!), and 1+F and 150K+ on a wet runway somewhere more limiting...
It basically tries to get the best (highest) Flex Temp, and presumably is "random" since it cannot understand rounding errors in it's calculations, nor take into Flap / Tyre wear, or having your fillings fall out at LIS at 155K when it would quite happily fly at 135K...