you have to be willing to cinch it up with just your nose hanging out.

This cannot be overemphasized. Especially for those who have pared every ounce out of their load and find they are at the limits of their warmth system.

If you are in a situation where you need every scintilla of warmth your bag can provide, you must cinch the head opening down as far as you can, while being careful not to trap the moisture in your breath. That means leaving a nose-sized hole and nothing more! Otherwise you are not utilizing your bag to its maximum warmth.