David asked 'Incidentally, I notice that the Pro medium format version of Tri-X is rated at 320. Is it the same emulsion or is it really a bit slower?'
320TXP is a different emulsion, with different characteristics, especially in the darker tones - it has lower contrast in the shadows than the midtones and highlights which gives smooth shadow detail that seems to go on for ever, good midtones and bright highlights. Because of that it's a film that responds to careful midtone placement more than other films. 400TX is more of a standard straight-line sort of a film. I'd say that 320TXP doesn't push nearly as well as 400TX.
I'm not sure what you are getting at. Nobody but you has referred to ASA in this thread - the only reference to a standard speed has been ISO 400 for Tri-X, which is what Kodak themselves say on the 400TX box. All other references are to an EI or to a simple number. What's wrong with that?