Blood and souls for Makhleb!
Henzell says...
  1. Spell success rates are coloured according to maximum {miscast} damage. Grey means 0, white means up to 10% of max HP, yellow means 10-30%, light red means 30-50%, red means 50-70%, and purple means 70+%.
  2. 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.
  3. Working spell failure calculator at 'https://docs.google.com/spreadsheets/d/1CpWuJMVbo5APnLU4o4bvf0cYFohjSlZEs5Gk-ueyNsY/edit#gid=857947'

Information based on master branch and the great LearnDB.
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