First of all, you should research in literature similar experiments performed with HepG2 or other hepatic cells and the drugs you need to evaluate, that will bring you an idea of the hepatotoxic range concentration of the drug. Then, you stablish a range from low hepatotoxic concentration, to a dose that inhibits a 70-80 % of cell growing, using concentrations with the same difference between them, so the experiment will be uniform.
If there is not information about similar experiments, you should try evaluate a lot of concentrations as needed of the drug until you get some information about the response of the cells.