I've been on a few of those sink streets too. Although some people
will say that I bring my own sink street with me on each flight.

From looking at your graph of altitude required to minimize the chance
of landing out it seems that the old rule I've heard of programming in
a 800 or 1000 foot get home altitude would allow you to program in a
slightly shallower Macready setting and still stay above the curve.
Then when you get into that last few miles where the curve starts to
drop you can speed up and finish at a lower altitude if you wish.

One nice thing in the Cherokee is that if you can see it you possibly
can't glide to it. I had several single digit glides at the Region 10
Contest. In fact John if you're looking for a river sink look at my
trace from Day 2 at Region 10 where I landed out 4 miles from the
finish. I don't think it was just the built in sink in the Cherokee
either as Dave Coggins reported seeing L/D's in the sub-10 range in
his Nimbus on the way back too.

When in Marfa i drew 10:1 circles which for the Cherokee was close
enough for easy math to the 1/2 your L/D method to make sure you can
make it home. Best I can figure that is about a Mc 9 or 10. I only
thought I was going to land out once during that week, see the old
adage of where there is big lift there is big sink.