To help you translate zmax into the limitations of your MOT chamber.
(i.e., if you have 30mW and split it equally into 3 retro-reflected beams, you have 10mW per beam. Field gradient is 4.8 gauss/cm.)
These calculations are not necessarily correct - however in Rb they correspond reasonably well with both experimental evidence and with much more elaborate computer simulations. For details see PRA 56, pp. 4055-4063. For source code used to generate these graphs see this. For the theoretical background of this simulation see J. Dalibard, S. Reynaud, and C. Cohen-Tannoudji, J. Phys. B 17, 4577 (1984).