When you are hit by a direct-damage spell, you have a chance to resist 100%, 75%, 50%, 25%, or 0% of the spell's damage, depending on your resistance score in relation to the level of the attacking caster. Your average percentage of damage resisted is the weighted average of your resistance percentages for each level of damage resistance (100%, 75%, 50%, 25%, and 0%). Using, as an example, a fire resistance score of 250 vs. a level-50 mage's fire spell, you would on average resist roughly 75% of the damage of that spell (as can be deduced by calculating the weighted average for the corresponding column in the example table below). To further the example, if you had fire resistance of 100, you would, on average, resist roughly 30% of the spell's damage.

Here is an example of how various resistance scores play out against level-50 spells:

Resistance Score 50 100 150 200 250

Chance to Resist Spell 15% 30% 45% 60% 75%