In my opinion, it is naive.
First of all, the average yearly temperature means nothing for indoor heating. Suppose this average is 20°C: it could be half of the year at 5°C and half at 35°. It's even more complex, for instance if cold days and hot days alternate they have very little impact. The heating is mostly needed when the temperature stays low several days in a row.
Then, there are external sources of heat, like sun or human heat. They can increase the indoor temperature for a few degrees. I worked for years in an office, facing South, with no heating even when it was freezing outside, and it was okay.
Finally, the formula does not imply what you said. In plain words, it means the energy (heat) is proportional to the mass and the temperature difference. The factor depends on the material. So it does mean that increasing the temperature of 13° instead of 16° uses about 23% less energy. But it does not mean that this much energy is needed to keep this temperature inside a building. Suppose that, with an external temperature of 7° (the average for my city for February) an insulated building at 20° would lose 5° a day. Then dT=5 in the formula above. Whereas the same building at 17° would lose 3° per day, so dT=3, a 40% decrease in the energy needed.
In my opinion, there are too many factors to give a simple answer. It know it's possible to estimate this for a single building, given a proper model of the weather and sun exposure, but it is hard to generalize from an individual response.