I got myself a Naniwa flattening stone for this very purpose and immediately regretted it.

It worked, but it seemed like it would clog up fairly quickly. In addition when I laid it on a flat surface it was not flat! I agree with Kamots that they would in time need to be lapped themselves. I came here to see what people preferred and the consensus was either what you are already doing or get a DMT 325.

In answer to your question I believe the coarse DMT is the 325 and the number refers to the grit size. I could be wrong. I have not got round to ordering one yet. I don't do sufficient honing to need it just yet. I would be interested as to how you get on as I have the same Naniwa 12K.

I have heard of people using them for honing too (I guess bevel set) but it seems that it might be a little course at 325.