My pleasure, just keep in mind I'm still a newbie too, but there's an abundance of knowledge on here.
I found the formula I was thinking about look for BillyBob58's post about 3 down it's for underquilts but you might be able to modify it a bit if you can estimate the value of having the ground under the pad and the rating. You'll have to experiment some but to me that's half the fun.
You might also think about something like these pad wings to keep your shoulders warm. It would be an easy DIY project.
Have a good one.