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I use Mejiro right after a 5k Super Stone and I get almost shave ready edge. That can't be a 5k edge.

Mejiro performs finer than the estimate you got. In Japan people do not talk about grits when they talk about naturals, there is coarse, medium, and fininishing grade stones.
The key is how Jnats...

Grab a Mejiro or Tenjo nagura, and follow with tomonagura.