This gets into a fuzzy area, since it's a multidimensional equation, and often, even the founders can't tell you what your share is "worth" without lots of caveats and time spent on spreadsheets. There's usually not a single answer.

The important principle, though, is that they shouldn't be hiding anything from you. If the company won't even tell you the percentage your shares represent, you can't trust them to do anything else.