That does also suggest the blowback scaling with spell level. Perhaps cantrips only do one point of damage on blowback, while leveled spells do 1d4?
Or 1d4 for levels 1-2, 1d6 for levels 3-4, 1d8 for levels 5-6, 1d10 levels 7-8, 1d12 for level 9 spells? Non-damage spells wouldn't change.
I...