I think a crude rule is to fly 1000ft above ridges for every 10kt of wind aloft, for any downdraughts to be smaller than 500fpm.
This has always worked for me when crossing the Alps for example, which I normally do ~ FL180 (on oxygen) which is ~ 8k above the peaks (on the relevant routing) and I have never seen any significant turbulence for wind aloft values of say 20-30kt.