Again, I'm no expert here, just trying to figure it out too, and I don't think there's a complete, consistent story here but...
Richard's formula is something like
CLO = (T - 90) * 0.08 / MET
where CLO is the CLO you need
T is the minimum temperature
the constants 90 and 0.08 I derived just by looking at the chart
MET is the metabolic level
Regarding CLO/inch for down, I have not found any consistent value.
Like I said, for Apex, it's 3.4 CLO/inch, using thruhiker's numbers. Some people say down has the same CLO/inch than synthetic and some people say it's less.
Because of the uncertainty of CLO/inch for down, It seems like all this analysis is not going to get you your final answer.
Maybe you're best off looking at down bags, finding several rated at the temperature you want, see how much loft they have, and use that.
What is the BPL position statement?
What is 3.5 - 0.05 * T? I haven't seen that. That would mean 3.5 inches for 0 F. From Richard's chart you need a CLO of 9.5 for 0 F. That would mean a CLO/inch of 2.7.
If you get answers of 2.5", 2.2", and 1.9" using different methods, that might be as consistent as you'll ever get.
I assume all of these assume you're wearing minimum clothing inside your bag. If you wear insulated clothing you can extend the range down 10 or 20 F.