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Howard --
I don't have any data for TMY/76 st., but I think it may be closer to 320 than 400. I'm satisfied that TMY in DDX+6 is an honest 400 combination so I'd recommend that combination as suitable for calibration.
I think you may be overexposing your test a little and I'd worry about losing some toe contour if the Plotter finds the speed point at 2.58. If we call that 2.6 (as a nice round number) and assume that TMY/76 st. is actually a 320 combination, then its speed point should fall on 2.3 (if your target ISO reference point is 2.4), and you should reduce your test exposure by 2.6-2.3=0.3 or one stop. In other words, stop down a stop and run another test.
There's nothing particularly magic about that 2.4 reference point; you can use any value that's convenient for you if its test conditions are repeatable. I recommend 2.4 because it provides enough leeway to include the toes of very fast (faster than their ISO numbers) films, while providing enough curve length to make the Plotter's analysis reasonably reliable. Another minor advantage of the 2.4 point is that it's the value I use for all the test data I provide so if you want to use my test files you should set the reference speed point at 2.4 in the Plotter (that's the default value).
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