LASSI is a two-step diagonalization method for recovering CASCI-quality results by restoring interfragment interaction to a wave function of the LASSCF type. However, to achieve CASCI-quality results without CASCI-like computational cost requires the intelligent specification of the space of model states constructed as products of fragment statelets. Here, by examining the convergence of systematic LASSI[r,q] calculations, we have identified that significant fragment statelets are usually those in which the total spin and charge of each fragment differ by up to ±1, and that explicit consideration of interactions between fragments should be accounted for in the optimization of the CI vectors of charge-transfer fragment statelets in order to achieve fast convergence. We have synthesized these insights into the method we name "LASSIS", which consists of a generic recipe for model space construction which proceeds automatically from the specification of the reference wave function without further user input. For the bisdiazene potential energy curve and for the aluminumsubstituted di-iron MOF node, LASSIS reproduces qualitatively converged LASSI[r,q] (and therefore CASCI) with at least one order of magnitude fewer model states.