It doesn't matter. The whole point of the IT8 target for calibrating your scanner is that your scanner reads colors the way they are supposed to be read. Otherwise your scanner may introduce a color shift that you'll then need to compensate for. All mask color compensation is done in the computer. The inversion of the image is done in the computer. Why wouldn't you want to start with the most true color image to begin with? Not calibrating your scanner is like shooting with a colored filter on your camera all of the time. I just don't get why it would work with slide film yet can't with negative film.
Give it a try, first try to find a negative IT8 target but I think you'll draw a blank (that should ring alarm bells) so you'll have to shoot one yourself.
The standard IT8 won't have enough patches to describe the colour space, so you'll need to have a custom one made or find one will a larger than normal gamut.
The resulting profile will only be valid for precisely the same exposure conditions as the target capture, including lighting colour, lighting intensity and exposure.
If you go this route you'll need to make one IT8 for each 'scene value' that is each different colour temperature or lighting possibility, the work will be extreme; the accuracy relatively low-possible but not worth the effort.
This is why Vuescan and Silverfast profiles for negative aren't very good, just ballpark.
Your main problems will be:
The orange colour correction mask built-in to colour negative films is not only exposure-dependent and different from emulsion-to-emulsion type, but also highly non-linear relative to exposure. Being produced from up to 12 separate emulsion/dye layers. The colour space of c-neg film is not a simple 3-D product, and vastly more complex and less uniform (in 3-D terms) than the colour space of transparency slide film or a digital camera. To characterise this increased colour space complexity, a negative film target will therefore contain many more patches than a normal scanner target.
Not impossible just really tough and you won't get any better results than Kodak's 'scene matching' method where they shoot a known target under a given light and balance that on final output with a colourimeter.
