Where should my sit bones rest on a leather hammock/sling saddle? Where is the ideal / designed location for my sit bones to push on a leather sling/hammock style saddle?  (Relative positioning, that works for any saddle width)

While I'm riding on my leather saddle, I notice that my sit bones seem to press the saddle very close to the rear metal part of the saddle (i.e. near the rivets).  This metal bar with the rivets is U-shaped, with the tails curving forwards at each side.  Are my sit bones supposed to fit within the curve?  In front of it?

Referring to the grid below, my intuition is that I should get a wider saddle with a wider metal U such that my sit bones are in the relative locations of: center-of (C-D,2-3) and center-of (C-D,4-5).  Am I correct that the U should "cradle" my sit bones?

(Also, I notice most leather saddles use 6 rivets in the rear, so we can refer to them as rivet #1 through #6.)
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.