Hi Ellen,
Sorry to say I have seen some very heavy weight only moorings move more then they should have in very bad weather.
If this mooring is going to be exposed in heavy weather with the boat on it I would consider an attached mooring. I know that is not possible some places, but needs to be considered. Otherwise I would put down as much weight as possible. I have never seen any formulas as bottom conditions would have a huge impact.
Ahoy Ellen , FWIW , I dove on my anchor, found it didn’t set well. I was amazed when I picked it up and moved it to a sandy spot . (things weigh less underwater)
What amazed me was how light it felt . A 35 lb CQR on the surface is heavy to move around on deck , but under water it only felt half as much weight and not hard at all to pick it up and move it, walking across the bottom .
The concrete blocks we used in Monterey Bay, CA., were 3’ X 3’ x 3’ blocks and they were set, in sand, behind the breakwater . The attachment loop was cast inside the block with 3 connected battle ship anchor chain links.