there will either be a relief valve at the main hyd. control valve or on the pump, (most forklifts it's on the hyd control valve) and the setting I would use would be the one where it will ONLY lift a capacity load and NOT lift a capacity load + 100 kilos.
I wouuld also check to make sure you are not pulling air in to the pump via the suction line, (is there foam in the hydraulic oil?) and make sure the suction hose is not collapsing.