Marty,
You're in the right ballpark. Bullseye also works well. My preference for the Red Dot stems from preferring to use a larger flake type of powder than some of the ball powders of similar burning rate. Some very nasty detonation type of phenomenon occur on occasion with severe underloadings of fast burning ball powders, so just to be on the safe side all these years, I've settled on either Red Dot or Bulleseye, and have never had a problem with the thousands and thousands of lap loads that I've put through barrels .
I've never tried clays as a lapping powder, so can't really give any insight there. I just thought I'd let you know that my choice of powder isn't strictly burning rate, but the other consideration as well.
Be safe, and let us know how your project goes!
God Bless,
Marshall