How to appropriately scale up cantrips players want to cast using spell slots?

How to appropriately scale up cantrips players want to cast using spell slots? Like, Ray of Frost with a 9th level slot, etc. Check spell design guidelines in DMG, lists typical damage by spell level. I'd use that.

— Mike Mearls (@mikemearls) July 8, 2017

So RoF would become 15d10 with 9th level slot? Or 60d10 since it’s 4x the damage at 17th (4d10 for cantrip) 15d10 – i'd use listed damage without multiplier

— Mike Mearls (@mikemearls) July 9, 2017