There are a few too many variables to give you a specific answer and at the end of the day its up to your personal preference and common sense to some extent.
If you can wear an extra jumper instead of walking around the house naked, you can turn your roomstat down. If you can tolerate cooler showers in the morning, you can turn your cyclinder stat down. The list goes on..........and on........and on...........
I dont think you will find anyone who is going to tell you it's 20.65 degrees.
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