Max drift = half the wind (100kt) or 1/3 of the wind (150kt). Anything more precise is wasted in the error in the winds aloft forecast.
Navbox Pro is the way to go for European VFR and "UK-style" (i.e. Class G mostly) IFR. You get 28-day updates and these come from the national AIPs, same as the vastly more expensive Jeppesen data (which I have used also). I am not aware of any other flight planning program for Europe which gets proper regular updates, and updates are crucial.