Bill Tindall, E.Tn.
what does one use a 6" stone for that it matters if it is flat? When would it matter if any stone was dead flat along its long axis? I'm trying to work out in my head the geometry. I can't come up with a reason why even an 8" stone used for a plane blade would need to be flat along it long axis.

Nothing I use for knives is close to flat. I think the result will be a convex edge which I think is a good profile.