Thanks for sharing. The ISO standard sets the speed point to 0.6 >b+f. Your data points tell a different story. They are pretty much all over the place. It hard to make out an accurate intersection point. Ilford's own product literature shows a much better fit to the ISO standard. So did (does) the Agfa and Kodak literature, by the way. My own data, also shows little to no deviation from the manufacture data or the standard. I don't know where your deviation comes from, but these are very rough data curves.

The difference might be that I judge the intersection point(s) after applying a best-fit curve through the data points. Best-fit equations for s-shaped curves are not trivial. I can share my empirical equations with you. They were evaluated by Ilford's Technical Department a while back, and they were impressed with how close the data fit was, especially at toe and shoulder. However, I don't think Excel can handle them. You might need a dedicated graphing program.

Different people taken similar measurements often get very different results. Potential sources for variation might be the spectral sensitivity of the densitometer. The red-content in toned or warm-tone papers was too much for my old Agfa densitometer and always gave skewed results. Other issues are often within the consistency of testing. Test exposures should not be collected and then develop together. That alters the results. Just a few thoughts, the list goes on as you probably know. Again, I can't tell where the difference comes from. Maybe you can share how you did your tests in detail.

However, for this conversation it doesn't matter where the speed point is, because it rarely is where we need it to be for an exposure-correction-free contrast change anyway. Nothing beats a well-made test strip.