"You can't rebuild all steps yourself so if a client wants rec709 you need to apply ARRI's 3D LUT. Since it requires a logC input you need to convert your linear EXRs to logC first which can be done using Fusion's CineonLog tool."
Hey, I just wondered if you expand on why you "Can't" rebuild these steps yourself?
The conversion from LogC to Linear scene referred can be done with a simple formula, then use the Gamut tool to convert to the particular Gamut that you are targeting, and also apply a gamma that is appropriate for the viewing system being used. I think I am missing the point in your post, I just wanted to make sure in my mind what you where meaning by it. :-)
However, as you stated you can download a 3D Lut that will do the Tone-Mapping, Colour conversion and Gamma correction all at once. This is easier than doing it yourself, but that is no fun ;-)
Linear conversion with no Tone-Mapping can produce less saturated RGB Values, perhaps this is part of the problem? I am going to look into that, I am now curious.
p.s. after some further reading, it was said that the formula was where the change happened. When using the formula, there are some variables that could be different or not taken into account, such as, was the encoding done in SUP 3.x or 2.x, the exposure index since ver 3.x can go from 160 to 3200 and version 2.x can go from 160 to 1600, and whether you are using normalised or relative scene exposure values. These can change the final result.
Wow I am an idiot for not reading this all the way through.... oh well. You can ignore p.s. #1