The logic needs to be something like this:

Outside 20deg - Inside heated to 20 deg then switch off.
Outside 10deg - Inside heated to 50deg then switch off.
Outside 0 deg - Inside heated to 80deg then switch off.

There also need to be an adjustable "shift" on the curve so that it can be
varied downwards to something like:

Outside 20deg - Inside heated to 20 deg then switch off.
Outside 10deg - Inside heated to 30deg then switch off.
Outside 0 deg - Inside heated to 40deg then switch off.


That is not a "shift"- it is a gain change. Your equations are
Tint=G*(20oC-Text)+20oC where Tint is interior temperature cutoff and
Text is exterior temperature measurement. Then G=3 in the first case,
and G=1 in the second. Rewriting the equation as Tint=(G+1)*20oC-G*Text
suggests- not to be taken literally:

Please view in a fixed-width font such as Courier.



|\
REF V ----------|+\
20o | ----+---V (setpoint)
+-|-/ | T
| |/ | int
| |
V --/\/\--+---/\/\--+
T R GR
ext