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J-nat should be interchangeable with Shapton 30k, what the finest grit is of a good J-nat is unknown.
For Coticules I have seen estimations in the range of 8-12k finish, but I am not sure how...

Well for what its worth you need more than one Jnat to be able to get best edge from any razor. Not every stone will work equally well with every steel.
8k is good enough, as confirmed by many, to...

Kevin,
I agree with you. I opt for palm size to get a better stone (as in faster and harder) for reasonable price.

So has stones for almost any pocket.

1 and 2 you have to test and see , all stones have their own character.
#3. this is not a stone, its a system that someone designed to rate stones ,that is not used by any other reputable seller,...