Mark Nowicki
I just flattened my water stones yesterday (Kings, 1000, 4000 and 8000) using the method I learned at a LeeValley seminar years ago; drywall sanding screen (220 grit), dipped in water and placed wet on something flat.

In my case it is a 12"x 12" granite tile on top of a granite counter top cutoff on top of an old towel on top of
of my bench.

I mentioned this method at a Lie-Nielsen travelling sales show, the rep grudgingly admitted that my method might be just as good as the $500 one.