I treat it like an outdoor plant and use an amount based on the surface area, just a good spread around, not enough to hurt the plant and if it weren't enough it can always be done again.
I agree. It comes down to how fast your ladies eat. Stronger light will get them to use nutrient faster. It's hard to tell you exactly what to use and how often. I like to give many organic materials, but in small doses. They all get seabird guano (12-12-5) through veg and up til mid flower. Then I switch to a combination of guanos that give me a mean NPK value of about 6-10-10. In late flower they get no significant nitrogen. Then they finish with no nutrient until they are sufficiently starved for a clean harvest.
All the while, however, they get random additions of bone meal, fish meal, kelp meal, etc... And in very small doses.