>>TBH, I've no idea what the average thermostat setting might be in Europe (or the U.S) . Mine's set at 12C (so roughly 53F) and that seems adequate for most of the house. A couple of rooms that are most used have additional heating (a wood fire and an AGA) but for bedrooms etc that setting seems fine. I must say I'd find 66 uncomfortably warm....
>>
>>I remember reading somewhere that reducing the setting by 1 degree can reduce your heating costs by 10%
>>But that's seems a stupid statistic - the obvious thing to do would be to set it to 70 then reduce it by 10 degrees at which point your heating costs should be approaching zero!
>
>Check your math - it should go down to 34.87% of what it was. Though I also doubt that the curve is that smoothly exponential.

I know - like compound interest in reverse. But the calculation can't work without taking into account the ambient temperature. eg:
Ambient temp :65. Turn down heating from 70 to 60. Save 65% .......