The “Tone curve” flag in the Color Management module is specific to each input profile, as are the Base Table and Look Table flags. I would suggest to try and find what settings give the best results for your input profile. The goal is to get the most faithful representation of the negative as the starting point for the conversion.
Then, after applying the conversion, you can play with the tone curves in the Exposure module to adjust contrast to taste, as you would do with a nomal digital image.
I don’t have a “scientific” method for adusting contrast: since the tone curves are such a wonderful tool, giving an immediate feedback, i usually just go ahead and “feel it” 
Regarding my recent example, where i only used a linear segment, that was just because of sheer laziness, sorry for that 