>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.