A First-principles Bottom-up study of (2,5-dimethylpyrazine)CuCl2 done at the UB3LYP level has shown that it presents a 3D magnetic topology that can be best described as a set of weakly interacting regular S = ½ AFM chains. The JAB value for the intrachain interactions is −9.2 cm−1, while the strongest interchain JAB interactions are ca. −1.9 cm−1. The 3D magnetic topology of (2,5-dimethylpyrazine)CuCl2 reproduces well the experimentally available magnetic susceptibility curve. The two largest computed JAB values are remarkably close to the experimental fitted parameters using a weakly interacting set of regular AFM chains model. Computed and fitting magnetic models do not agree, but are able to reproduce the experimental magnetic data. This raises doubts on the accuracy of determining the dimensionality of the magnetic interactions in solids by fitting the experimental magnetic susceptibility curves. An in-depth study was then performed on how the χ(T) curves vary when the magnetic topology of a solid changes its dimensionality nearly continuously from 3D to 2D and, then, from 2D to 1D. This study illustrates the relevance of computational chemistry methods as tools to guide synthetic chemists in choosing the right physical magnetic model for their systems.