I think most of sex developments are determined by genetics. But ambient conditions can influence those seeds that arnt well determined before and reduce hermie risk.

Suggestions to increase female ratio:

-Light spectrum and intensity: enought blue light, cold fluos (865) or MH. And provide good intensity after the first week (lights near, specially floros, and with reflectors).

-High humidity level. Never below 60%, 65 to 75% is ideal.

-Enough N. How achieve it depends of substrate, fertilizer, etc

-Avoid too much heat and excesive drying.