In theory, you should set the WB to the exact color temperature of the backlight (either measured by your camera, or by picking a spot on the flatfield from RT) and forget about it, because this is what your custom profile was calibrated for.
In practice, since we don’t know the exact color profile of the negative, altering the input WB can be a clever trick to slightly alter the input profile (and thus, the output of the inversion), in ways that could not be possible by simply adjusting the exponents or the output balance multipliers.
Think of what happens mathematically: for each raw channel, we are computing a weighted sum of the 3 channels, raising the result to an exponent, multiplying by some factor, and that’s our result in working space.
By changing the input WB (which happens before the input matrix) we are in fact also changing the weights of the input matrix.
So… yes, this trick can be put to good use, but it can also get confusing. If you want to adjust colors very precisely to your taste, RT has plenty of solid tools to do that (like L*a*b Adjustments, HSV Equalizer, Color Toning, etc.), i would suggest to use those instead 
(see below)
in this version, yes it is ok! 
Before, in RT 5.7 and 5.8 you had to re-balance each time you changed exponents. Now, the spot that you choose with the “Pick white balance spot” button, will be saved as an “anchor point”, and the multipliers will be automatically adjusted so that the same input will always produce the same output, regardless of the exponents.
Unfortunately for now, the only use i can suggest is to eyeball the result. 
You can make the task a bit easier by using the Lockable Color Picker feature, and compare the values with the “official” color values of the target (which you should easily find online). Maybe the most effective way to evaluate color is to use HSV coordinates.
Rec2020 it will be
Very good idea! Thanks for that, i’ll change it.
You we both right: different exposures require different exponent ratios. Here’s an example.
I have a roll where most pictures work pretty well with the default settings. I copied the same settings (exponent/ratios, reference input values, output level/balance) on all frames, and all were digitized at the same exposure setting, so channel multipliers are consistent. This way it’s easy to evaluate whether a frame is over- or underexposed with respect to another.
Well, i have this frame, which is clearly overexposed. This is also evident by the naked-eye appearence of the neg:
Let’s say i want to recover it: i simply lower the Output Level, and i get this:
The highlights, that were previously clipping to white, are now somewhat pink. I can try to adjust the output balance to make the facade gray, but now everything gets greenish
This is because (i think), as the film approaches Dmax, each channel exponent starts to diverge from the nominal value. This is not explicit in Fuji and Kodak datasheets, as the curves are abruptly cut:
but i’m pretty sure if the manufacturers were to trace the curves further to the right, we would see a knee on each curve.
Anyway, to fix the picture i can now lower the Red ratio (which means lowering the red channel exponent, since red exponent = red ratio * reference exponent), and raise the Blue ratio a bit, until the subject becomes neutral gray.
Note that the overexposure of the film also makes the picture less contrasted. I could further raise the reference exponent, or tweak the tone curve … you get the point.
Also notice how the lower part of the picture, in shadow, now has a slight reddish color cast. That’s because the new exponents are not good for normal exposure levels; we are sacrificing the shadows in order to recover the overexposed part.
For reference, the values here were:
red ratio = 1.151
blue ratio = 0.93
In general, try to experiment with your overexposed negatives: you will be amazed at the amount of information that’s captured in there!
Yes, you nailed it