A technique using two measurements from opposing directions for the assay of the activity of radioactive waste drums, mainly consisting of organic materials, has been studied. A model for the calculation of systematic errors is given by simulating the measurement system. The calculated values are in good agreement with the experimental values. This confirms the validity of the model and proofs the good performance of this technique in practice. These results may provide guidelines for setting up a measuring system for the assay the radwaste drums.