The DUSM method is described for constructing the single-shell states and the corresponding matrix elements needed for describing the isotopes of a given nucleus within a major shell in the framework of the nuclear shell model. The advantage of this approach is that a streamline set of states are constructed, thereby reducing the calculation requirements. The idea is to truncate the full space of permutational irreducible representations with two columns to subspaces where the second column is restricted in its number of boxes - depending on the isospin value. The usefulness of this method for large-scale shell model calculations in the fp- and sdgh-shell is emphasized. An application to Sn isotopes in the sdgh-shell is presented.