What is the "Nikon Flat Picture Control"? And how does one access it via LR or ACR?
The only Nikon Picture Controls I am aware of are those selectable in-camera and which apply only to the jpeg image.
In the Camera Calibration tab in ACR. If you open a JPEG, then in the
Camera Profile name it will use
Embedded. But if you open a RAW file, it will list
Adobe Standard and all the picture controls you can set in the camera or any other profiles you have loaded into ACR/LR.
Most people pass over the
Camera Calibration tab which is a big mistake. It probably should be the 1st place you start on a file, especially a RAW file. All the other controls become calibrated to that initial starting point that is set by the
Camera Profile. You can also adjust the color balance of the Shadows, Greens, Reds and Blues. For example, I find Nikons tend to over saturate the greens and Canons over saturate the reds. So you can adjust these for your camera and then save them so ACR starts using that profile.
This is also where if you have a Color Checker, you can shoot it in different light and you can build camera profiles to adjust the image out of the gate to a better state and then only have to tweak it instead of having to make large moves. Again, with JPEG files, you are much better doing an in camera white balance since all the color data is hard coded into the JPEG file already.