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Dear Sir:

ISO setting on most of the cameras, D600 being among those, is acting similar to microphone amplifier; that is, the signal is captured before the ISO gain is applied. In such situations most of the noise and non-linearities are already baked into the signal, and the gain does not help much when it comes to decreasing overall noise. 

Suppose the camera at base ISO setting allows for 10 stops of photographically usable dynamic range. Increasing ISO by 1 stop, we will be clipping 1 stop in highlights (provided exposure settings, that is aperture and shutter speed are the same; and the light did not change between the shots); but we will be getting a tiny bit more in shadows, typically around 1/6 EV for D600. If we decrease the exposure (again, only shutter speed and aperture are parts of the exposure here, ISO is not) to prevent highlight clipping - and that means that we are going down at least 1 EV in exposure - the total amount of light recorded is decreased, and we will be plugging shodows by close to 1 stop. Net result - whatever we do, increasing the ISO setting by one stop we shrink the total recorded dynamic range (either clipping the highlights, or plugging the shadows, or both) by close to 1 stop.

In my experience ISO 800, as you suggested, would be a fair estimate for the point, where one needs to start decreasing the underexposure limit. For my purposes, I prefer to start decteasing it from ISO 640, as I print large.