A programming library was developed, based on Stratton-Chu diffraction integrals for calculating reflected optical fields. Dipole-type focusing schemes with tunable number of beams and mirror placements were studied, considering the influence of phase distortion and aberrations. The intensity above $3\times 10^{26}$ W/cm$^2$ was found theoretically attainable in a system of 12 beams of 50 PW each with about 90% of that value realistically achievable.