Turbulence is both subjective and dynamic. The best you can do is use what info you have when you plan the flight and either plan lower or give some additional fuel for altitude adjustments. If a flight is weight and fuel tankage critical, I generally dont adjust the altitude or give extra fuel unless the turbulence is continuous moderate or greater.
As a dispatcher, your job is to operate flights in barely legal and close to unsafe conditions. If turbulence planning makes you nervous, just wait for some of the restrictive MELs you will have sometimes, the thunderstorm days where no route is really that good, morning fog with a bunch of high mins captains and CAT I only planes, or the international flights where the destination is forecasting a typhoon. You can add on to all that planes are packed so heavy today and put on routes at the edge of their range that even if you dont have a strict fuel policy to reduce extra fuel, the payload and tankage will often reduce you to very little extra fuel on top of the FAR requirements.
You are a risk manager/risk assessor as a dispatcher. Your job is decide when a barely legal forecast or actual conditions are safe to operate in. Its your job to decide when you need to offload payload for the fuel that you or to fuel stop the plane enroute.
