The table below casting. The first TN on the table is what you need to partially cast the spell, the second TN is what you need to fully cast the spell.
The columns of the table are effected by your level. All casters get better at casting spells as they level up because they move farther to the right on this table.
The rows of the table is how much will you invest into the spell. Casters can either invest themselves Minimally, Significantly, and Totally to the casting of the spell.
As you can see, the more Will you invest the better the odds of your spell being successful. However! If you look down at “backlash”, the more you invest the more you’re risking! A spell you’ve invested all your will into that FAILS causes a total disaster! Whereas a minimally invested spell has no effect, but is not likely to succeed.
Does this help?