In addition to the Chart I'm working with in order to create the spells, I've got a list of mitigating factors for the spells. These won't change the TN for the spell but it does change how much Mettle is used. These will be rated -1, -2, etc. based on how big a deal they are. If the spell has a specific time it has to be cast, 'midnight' might be a -1, reducing the mana cost of the spell by one. If the spell had to be cast at the full moon of the fourth season of a year, it would probably reduce it more.
Rare Component - If the spell requires a material component that is rare or important.
Specific Timing- If the spell can only be done at a specific time.
Glamour- If the spells effect is only an illusion. (The rationale is that its easier to make something that looks like a brick wall than it is to actually make a brick wall.)
Focus- If the spellcaster uses a special focus. staff, rod, ring, book, etc.
Sypathurgic link- Sypathurgy...
Maybe I'll talk about that next time....