This paper presents an improved subdomain multilevel approach (SMA), a technique efficiently used for solving large complex-shaped antennas printed in multilayered media structure. In a large electromagnetic problem, where the memory occupation and computational time have been already significantly reduced using the SMA, the calculation of the macro basis function (MBF) reaction terms remains the most time-consuming part of the procedure. In this paper, we present an efficient way of computing MBF reaction terms and reducing them to equivalent moments. The numerical results are compared with brute-force computations, the SMA, and the measurements. They demonstrate conclusively the efficiency and power of the new proposed technique.