This sounds more to me like something where you would do your calcs (Ave and StDev and 1*sigma off Ave) in Table A, then post the result in Table B.
I did pretty much the same thing as you did in your file, but it doesn't work...
So I started over and it works now!
It happens to me very often that a thing doesn't work in Filemaker, but works when I delete and redo it...
Thanks for your time any way