I think you are on the wrong track here.
First, sensor dynamic range is an ill-defined concept. Sure, you have the number of bits recorded from the sensor, and you can simply take \log_2, but that does not map to anything useful since the last few bits are surely going to be noise, which is a big problem in the shadows because relative to the signal, the noise will be overwhelming.
Second, what you are deciding in practice is where the contrast should be distributed globally. You are choosing a mapping f: \mathbb{R}^+ \to [0,1], the slope of which (or f^{-1} if you want to think in display terms) is the contrast. The convention is that middle gray is aligned with the most interesting part of the image, so its gets the highest slope (contrast), which worked nicely with chemical film, but that’s just a convention, you can change that in digital photography.
filmic rgb’s has a bounded f^{-1}([0,1]), while sigmoid maps the whole interval, but away from middle gray both have a tiny amount of slope anyway. In some applications it makes a subtle difference, but you have to be careful: since f is bounded and increasing, you only have so much slope to go around. In other words, if you want contrast in highlights and/or shadows, you have to give up contrast in the middle.
It is really an artistic choice. The best practical way may be deciding where in the tonal range your subjects are, and working back from there. Eg for a particular image, it may be that you want contrast in the midtones and a bit in the highlights, and less in the shadows. In sigmoid you can do this with the skew parameter, in filmic rgb the white/black points.