A method for adjusting an on-time calculation model or a lookup table used for determining on-time values for an injector of a cylinder in a combustion engine useful in a motor vehicle, where the injector is connected to a fuel accumulator. The method comprises the steps of injecting a test fuel quantity exceeding the maximum allowed fuel injection quantity under such conditions that no ignition of the injected fuel takes place; measuring the pressure drop in the fuel accumulator during the test injection; establishing the actual fuel quantity injected during the test injection based on the pressure drop; performing a comparison based on the actual fuel quantity or the corresponding on-time value established with the aid of the on-time calculation model or the lookup table; and adjusting the on-time calculation model or lookup table in dependence on the result of the comparison. An apparatus which performs the method. A computer program to perform the method.