Measurement of drainage relative permeability by the centrifuge method was first introduced by Hagoort (SPE J. 29(3):139-150,1980). It has been shown that capillary end effects can cause error in the measurement of relative permeability if a minimum rotational speed is not honoured. To determine the minimum rotational speed that makes the capillary end effect negligible, (omega)min, we propose that the value of capillary-gravity number, N-eg, should be of the order of 10(-2) or smaller. This conclusion is based on the use a Forward-backward scheme consisting of a forward numerical simulator developed for centrifuge experiments and applying Hagoort's method as a backward model. The article presents the use of this Forward-backward scheme as a powerful tool for error analysis such as determining the impact of capillary end effects. By using this loop, we first determine (omega)min for specific core and fluid properties. Later, we generalize the (omega)min calculations by using the definition of N-eg as a "rule of thumb" for designing relative permeability experiments by centrifuge method. We also demonstrate another use of this loop for controlling the quality of the experimental data.