This paper proposes a novel methodology for calculating a space-varying permeability in HTS bulks, ensuring the same magnetic energy stored in E-J models at each infinitesimal volume, for the electromagnetic force calculation in HTS bulks.  The proposed method was validated using simulation and experimental tests based on YBCO/GdBCO permanent-magnet force interactions. These results show that, with a previous general study of the geometry to optimize, the equivalent model can be used for optimization purposes, greatly reducing the computational time.