Its a case of experience and guess and check until the ratio gives good separation. And agreed, benzene is a poor choice. For a reason beyond health ones, toluene and benzene are both bad because they absorb well in the UV and make it harder to read your plate by UV light. The most common combination is ethyl acetate/hexane. Starting with 1:3 EtOAc/Hexane is pretty standard as motoball said.