Spell DC Progression

The Difficulty Class to resist spell effects is 10 + 1/2 the spellcaster's caster level (round fractions down) + stat mod.
For example, the DC for a Fireball cast by a 5th level Wizard with 21 Int is 17 (10 +2 caster level +5 Int mod).
The DC for a Fireball cast by the same Wizard at level 20 with 26 Int is 28 (10 +10 caster level +8 Int mod).