I'm not sure where that math is coming from. If a character casts a 5th level spell, and that metamagic goes off, they get 5 points back. They need seven to turn it into a slot, which means they're getting 5/7ths, or 71% of the slot back.
I'm not suggesting a sliding scale for how many points you get back from a spell. Make it half the level of the spell. End of math.