I've discovered that in C1 you cannot obtain true linear Tone Curve.
Even you choose Linear curve and put Contrast Slider to -50 value C1 keeps to use some S-shaped curve.
Using this chart http://cl.ly/3F2v0T0j2x3P/RGB_255to0_52steps_Bayer.dng.zip
you can expect linear increase of brightness (all patches are graduated with 5 increment - 0,0,0; 5,5,5; 10,10,10... 250,250,250; 255,255,255).
What are you trying to achieve?
A contrast curve with a non-zero setting is definitely non-linear, very much by design. A negative exposure, however, is.
If you are just looking at the output values, your values depends on the output profile. There are no standard profiles in gamma 1.0, which would be quite wasteful.