If you're trying to find the "perfect" thickness you might be out of luck. Even in my very limited experience wood is very variable - I've found that adjacent slices from the same piece of timber can flex very differently from each other at the same thickness, enough to be noticeable by finger pressure.
I'm now working on the theory that I work the top until it "feel" right when I flex it, and different parts of the same top require more or less work to achieve an even feel. I believe that some builders use deflection jigs to check the board at different point, which I suppose to be the precision version of what I'm attempting by feel, but they also write that each top is different.
I'm not sure you'd want and "even feel" across the entire sound board. Some areas need to be stiffer, others need to flex more.
Thank you all for your replies. Though I am still at a loss as what to do. My idea, which I did not explain fully, was to be able to gradually thin the top down, testing it on this jig for the start of deflection and then brace it just enough so it would not deform under normal string tension. Perhaps not a well thought out plan. But I have one soprano I built with a very thin top and it has a wonderfully big sound but the top looks like a great design for a roller coaster.
If there are any further thoughts on this issue I would be glad to hear them.