I would start your experimentation at the formal midtone value (Gray card 18% or Zone V), using published box speed. Not all color neg films
are the same by any means. For example, Portra 160 might get you something down in Zone II, but Ektar probably won't. If you do have to guess, it's better to make an error toward overexposure, provided you do so conservatively. You also have to be aware of color balance issues
in color neg film for nonstandard lighting, which can depress one particular component dye in an imbalanced manner, making it impossible to
correctly reproduce relative to the others. In other words, use filters to correct for significant deviance in Kelvin temp from standard daylight
(like excessively blue shadow conditions). Some color neg films, esp those designed for amateur use, have quite a bit of latitude. But something like Ektar requires just as much care as when exposing a transparency, yet with maybe a stop more range each direction.