So I think that I've been looking at this particular forest from the inside of a hollowed out tree. After not quite liking my finished product I decided to look around for other examples and found one in the 2nd Ed FATE. (I think they are updating that site so it may be a bit weird.) It uses the same method as the one I was using, but executes it much more elegantly, so I've tweaked and adapted it for "Spider".
To create a spell (OOG), run through the list below adding the numbers together for what you want the spell to do. (Choose either Area or Targets not both) That number is the amount of Mettle you need to cast the spell. Add seven to that number, and you have the TN for the casting check.
convenient / inconvenient...1
Damaging / Healing...2
Incapacitating / facilitating...3
I'm formulating another chart to determine amount of damage. The TN for the spell will be the same, regardless of damage. However you will have to pump more magic into the spell using more of your mettle. So two spells might have the same TN but one might use 2 points of Mettle while the other uses 5.