Not disagreeing with you. I was simply describing a common approach to exposure index determination. The legitimacy of it is another story. Further, even if 0.1 above Fb+f is "correct", depending on how you test for the exposure that produces that density, you can get all sorts of exposure index values. My approach to film speed is somewhat different (as we've discussed before), although it may or may not intersect with 0.1 above Fb+f depending on the case.
I'm not sure I'm following the dichotomy of film speed and exposure. Why wouldn't they be different?