The advent of ALMA, the Atacama Large Millimeter/submillimeter Array, revolutionised our understanding of protoplanetary discs. Large disc samples observed at intermediate to low resolution became readily available to test long-standing disc evolution models: the viscous and MHD-wind scenarios. However, understanding populations of discs requires synthesising populations of models. When it comes to CO fluxes, one of the most easily accessible observables, running computationally-expensive thermochemical models for hundreds of discs can be prohibitively time-consuming. Here we introduce an alternative method based on modelling semi-analytically the disc CO fluxes. This method is fast, flexible and accurate: compared to more-complex physico-chemical models, we recover a good agreement within a factor of three. In Zagaria et al. (2023b) we discuss how these models can be used to understand better the currently-available data in Lupus and Upper Sco. Our method shows how powerful CO fluxes can be when studying disc evolution!