Sachiya,

I am not sure if the Naniwa flattening stone is the composition same as the Norton flattening stone.

If it is then 160/220 grit silicon carbide paper, used with soapy water on a peice of flat glass worked very well for me

As you have pointed out, the hard part is figuring when to stop !

I no longer use the Norton flattener, as I got a DMT 325 which Ihave found to be a much cleaner & easier way to lap hones.

Have fun !

Best regards

Russ