I am struggling to get my head around the right approach to building a tomonagura slurry. I understand that this should be used as the bridge between the final nagura slurry (probably either mejiro or koma) and just plain water without any slurry.

In reading up I have found several suggested options and would be grateful for some clarification:

1) A small separate piece of the same finishing stone. The slurry generated is obviously the same as the finishing stone and has the benefit of not leaving scratches (on the stone)

2) A small piece of a harder j-nat. I assume would act the same as above, i.e. slurry is from the primary finishing stone and doesn't leave scratches

3) A fine diamond plate / card such as DMT1.2k or Atoma. Again, slurry comes from primary finishing stone but can leave scratches. I know there was some recent debate on the impact of the scratches and that So uses this technique

4) A piece of another softer (but also very fine) finisher. I assume the slurry would come from this "nagura" and not the primary finishing stone and is therefore just a very fine nagura (like koma etc). I understand that this is what Stefan (mainaman) uses from his recent videos although I saw a post on foodieforums where he mentioned using a diamond card

Option 1) would appear to be the best but how many people actually have a separate small bit of their expensive j-nat finisher?

Thanks in advance for the guidance.

Claude