I got lucky with my Norton Flattening Stone that you see directly below. There's a 50/50 chance that it will NOT be flat, and it's good for Norton and softer water stones ONLY and you must use it before soaking the Norton water stones and under luke warm tap water. Naniwa also has their flattening stones and from what I've been told, they're worse than the Norton flattening stone is in terms of being flat and performance. Knowing what I know now, I will NOT buy this stone again:
Here's the absolute BEST thing you can buy period along with a 12 by 12 inch marble or granite floor tile and turn it upside down. Take the powder you see below and make a paste with it, and you'll have a the fastest and best water stone flattening system in the world:
^ Once I start getting all of my stones in including my naturals that I do plan on purchasing, I'll do a blog post and demonstrate how to flatten the stones using the system I described above.