There are so many variables that you're really going to need
to experiment to determine your own requirements. I keep a
"Temperature List" that I'm constantly refining in light of
new equipment and experience. It's based on Ed's list, and
goes in ten-degree (F) increments, listing the gear I need
for each range.
Personally, I don't drop down to a single layer of 3/8-inch
foam until it's 45-55 degrees F. Lower than that, I use Ed's
Pea Pod bag and combinations of pads.
Don't forget accessories for your head and feed. That's
where I lose a lot of my heat.