Almost everytime I have been cold in the backcountry the problem was too little insulation on the bottom.

The second problem is a hat or balaclava.

The third problem is I did not eat right.

The fourth problem is I over extended myself physically during the day.

I have not used a mummy bag in so long I am not competent to answer.
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"In theory, theory and practice are the same. In practice, they are not."
Yogi Berra