The combination of Lewis acids and Lewis bases, currently defined as catalysts and co-catalysts (or promoter) respectively, in the reaction between epoxides and CO2 to give cyclic carbonates, is discussed, starting from examples in which the Lewis base was used in larger amount with respect to the Lewis acid. In these cases, turnover frequency (TOF) values have been usually calculated taking into account solely the amount of the Lewis acid employed. The occurrence of two distinct reaction pathways, one catalysed by the sole Lewis base and the other one catalysed by the Lewis acid/Lewis base couple, in which the Lewis acid alone does not play a catalytic role, should bring researchers to reconsider the TOF calculation based solely on the amount of Lewis acid, especially when the amount of the Lewis base used is much larger than the Lewis acid. In order to draw a more precise picture of how the use of a Lewis base/Lewis acid co-catalytic system affects the reaction course, we analytically examined a general mechanistic scheme able to model the overall process. For these cases, we propose a simple equation for a better estimation of TOF values and a normalization approach for a more consistent comparison between various catalytic systems.