•Base chance is 10%, each level of the skill gives 3% extra.
•Target fortress percentage gives a bonus of -25% to 25%.
•Owner NP / Target NP gives a multiplier between 0.25 and 2.
•If target has no units, you always succeed.
So, if you use it with skill level 3 from 6M NP vs. a 20M NP mage who has low forts:
(0.1 + 0.03 * 3 + 0.25) * 6/20 = 0.132 -> 13.2% chance to succeed.