OK so heres an idea.
What if the amount of Mettle (manna) used was not a function of the amount of 'spell points' used to create the spell, but simply the result of a die roll.
It could look like:
If the Spell TN is < 13 than spell uses 1d6 worth of manna.
Spell TN 13 - 15 than spell uses 2d6 worth.
This would make spell creation faster AND mitigate some of the Mettle loss from a failed spell.
If you are casting a spell with a TN of 13 and you roll a seven (3d6 result: 3,1,3) then take lowest 2d6 (3,1=4) and lose 4 manna points.
Just a thought....