[en] In introductory reactor physics courses the four-factor-formula and two-group theory is normally used to demonstrate to students in a quantitative way the physics of nuclear reactors. However, the agreement obtained between lattice calculations with such simple recipes and measurements is not very good. This is in particular the case for light water reactors. To improve this situation it has been investigated whether it is possible by relative simple modifications of such a recipe to obtain better agreement between theory and measurements. The results obtained from this investigation are presented in this paper. It is seen from the results that significant improvements in the agreement between measurements and theory have been obtained. This report was presented to the meeting on Reactor Physics Calculations in the Nordic Countries, Helsinki, May 8-9, 1995. (au)