My cold weather (-20C, -4F) sleep system is comprised of a 63 cm * 196 cm (25" * 77") Prolite 4 (R3.2), space blanket (shiny side up) and a WM Puma DL.
If it gets much colder than this, sometimes, but not always, I will add a 1 cm (1/2") Zote foam pad (R1.36) between the space blanket and the Prolite 4.
If I'm not in a tent I will use my bivy that has a Refeltex bottom in lue of the space blanket.
I always wear something to bed - boxers and 'T' shirt or Merino wool long johns or more.
Here is another source for sleeping pad R Values