The baseline measurement would be the historical average values for a household in your area, maybe of your size.
And by the way I cut from >2x normal to 0.5x normal for electricity by that metric. While adding two kids to the house.
Halving again would be relatively easy in good housing stock such as PassiveHaus, but I have the house that I have for now.
Actually I *am* aiming to make it possible to reduce heat demand (again) by a factor of two with my FOSS 'smart zoning' project for which I have a small trial running this winter to see if my ideas stack up. The aim is to in fact improve comfort at the same time.
I *am* suggesting that people using (say) 4x the mean per person pay (say) 10x or more per unit, a little like the TEQ (Tradeable Emissions Quotas) concept, and I'd not necessarily have a cap (ie the multiplier continues to rise with total amount used) to ensure that even the top 1% would notice.