Blood and souls for Makhleb!
Henzell says...
  1. Spell success rates are colored according to miscast severity. Red means pretty bad things will probably happen to you!
  2. These entries describe the required skills to get a spell's success rate up to 55%. Try ??success_rate_N to get information on success rate of level N spells. skills_rate is 3*Spellcasting + 2*Intelligence + 12*(Average of skills in spell schools used by that spell).
  3. All success_rate_N entries assume: no worn armor with EV penalty, no elven or orcish body armor worn, not wearing a shield, not in bat form, spider form, or with blade hands.
  4. Note that Intelligence stat affects spell success 1/6 as much as average spell skill, and Spellcasting skill affects spell success 1/4 as much as average spell skill.
  5. Older versions used a different way of showing spell success. Success rate adjectives: useless 0%, terrible 0-5%, very poor 5-30%, poor 30-50%, fair 50-70%, good 70-80%, very good 80-90%, great 90-95%, excellent 95-99%, perfect 100%.

Information based on master branch and the great LearnDB.
© Dungeon Crawl Stone Soup, 2010.