Matt,
I don't know what "interpolating" means
What I did was add one drop at a time, then mix as per directions. At drop #8, the colour changed from blue to green. From experience, I know that the next drop will turn it yellow. So I am guesstimating that the real dKH is 8.5 from the intermediate green colour.
Maybe that's "interpolating"?
Ruth,
I have no idea what is precipitating out of the saltwater mix after 36 hours. I don't test my NSW, and am not really that curious to know what precipitates out. I just know to use it within 24 hours after salting it. Sorry I can't be of more help here.