Really Cantrippin'
Prerequisites: Cantrippin', Caster level 6th
Benefit: You make sacrifice a usage of a higher level spell slot to improve a cantrip. The DC of the cantrip is treated as though it were a spell of the spell slot's level, and any effects are multiplied by twice the spell slot's level(ex. a 1d3 damage-dealing spell deals 6d3 damage under the spell)

The way I read it its based on the spell slot you sacrifice.