Frank,
Yes, I agree with your placement on TriX. Sometimes I even place my low value at Zone IV with TriX. Depending if I want to give an impression of great luminance in the scene. Of course, the other thing that enters is what EI one rates a film at. I have found through testing that TriX (prof) actually exposes in my system at an EI of 160. This gives me a zone I density of .10 density above fb+fog. The reason that I mention this is that if one person rates TriX at 320 and is giving a Zone III exposure for shadows and another rates it at 160 and gives a Zone III exposure, then the second is actually giving a Zone IV exposure based upon the first persons rating of 320.

I think that John Sexton was once purported to have said "Nothing lives on Zone III". I have always observed a sense of luminance in his images. Perhaps that is why.