Where should my sit bones rest on a leather hammock/sling saddle?
You are correct about the position of sit bones between C-D 2-5. 
If the leather on the saddle has stretched, even if you rest your sit bones in the C-D 2-5 region, your sit-bones might be digging into the metal part. 


Measure your sit-bones distance, reference to another SE question 
If your sit bones are narrower than widest part of the metal bracket (about C,D 1-6) then tightening the saddle bolt will help a great deal.
If your sit-bones are wider than C 1-6, then tightening the saddle may not help much, you may have to consider getting a wider saddle.