@T70 The first post of @mikesan perfectly explained how RT works for soft-proofing
@mikesan I confirm that the output profile does not affect soft-proofing. For those who can read the code.
But in RT apparently the output profile isn’t used as the source profile for soft proofing, which seems a bit odd but perhaps useful in the context of overall RT functioning.
Do not think that the developer of the softproof feature (i.e. me) is a master in color correction. That’s why I asked Marty Maria (developer of lcms2) to jump in our discussion in issue 3406. I’ll emphasis 2 things he said :
To implement proofing, lcms as most CMM does this chain:
Input image → Input profile → CIELab → Proofed [printer] profile → CMYK → Proofed [printer] profile → CIELab → Monitor profile → RGB
There’s no place for the output profile in his typical chain, that 's why I’ve done it that way. I’ve first tried to make a double conversion : Working → Output (including change of bit depth) → Printer (soft-proof) → Monitor. Marty replied that bit-depth was part of the hard-proofing process, not soft-proofing.
The second thing is :
BTW, using softproof to preview the effect of working spaces or gamma curves is pointless. The use for soft proofing is to emulate printers. To some extent it may work for other usages, but this is not the goal of this tool.
Gamma curves can be changed to the user’s convenience for the output profile, hence my question to Marty on how to deal with this. his answer is that’s it’s not the purpose of soft-proofing.
Your advice are of course welcome, but at some point I need to see the light in the night of color management 
.
As for the values from the Navigator and the Color Pickers, they are either the one from the Working profile (direct conversion from Lab to Working profile (e.g. AdobeRGB)) if checked in Preferences / General, or the Output profile (direct conversion from Lab to Output profile) if unchecked.
Do not confuse between :
- the initial color space (PCS ?), which is Lab when the image is fully computed, just prior to any color conversion. This define the characteristic by which a color is represented
- the Working profile (e.g. AdobeRGB), which is a profile used by some tools for gamut handling (e.g. Chromaticity) at their very own step. This represent a limit (gamut) in the color space.