Like Tom and Paul, I use my possum down beanie with a Seirus Combo Clava. If I feel the need for more protection, I have a Sherpa ear flap hat to go over all and it will tie under the chin to hold in place as well as my goggles to protect eyes from wind.
As for the rest of the layers, inside out, Marmot Midweight Zip LS and Marmot Midweight bottoms, possum down socks with polypro liner sock and possum down gloves with thinsulate liner; Cocoon Pants, Cocoon Pullover, hat configuration above; Nunatak Arc-Alpinist, Thermarest Prolite 4 short, Bibler Winter Bivy. I used this 2 weekends ago when it got down to 5 degrees and I still was cozy, so I suspect it could bring me down to 0 F or just below and remain comfy.
Of course I keep clothing stuffed around under legs as padding and to make sure it's warmer when putting on in the morning. I will also have a couple hand/foot warmers close by, just in case I need to place by armpits and groin area to keep core thermoregulation up there. Like Cody Lundin’s book suggests - “98.6 Degrees; The Art of Keeping Your Ass Alive.”