We've spent a great deal of time trying to determine grits for our stones. The manufacturer has also worked with us, to no avail. A guess seems to be best.
Most abrasives are measured by the grit size used in the matrix. Our ceramic doesn 't work that way. Grit size is constant.
We've tried to compare scratch patterns as Cliff mentioned and this is probably the closest, but nothing that we can say "This is blah blah". Then the Japanese water stones jump into the equation and suddenly there is whole new set of numbers.
So where we end up is:
Our diamonds are a 400 mesh (measureable). (600 on the Duckfoot)
Our gray stone is "medium". (Same material as fine but different carriers and heat treat).
Our fine stone is fine.
Our extra fine is a surface ground fine.