Pieper. "The Four Cardinal Virtues", under humility, which is under Temperance. I'm almost certain he discusses it in "On Hope" also. Pieper was the one who opened my eyes that only a humble man can be Magnanimous.
Humility is you being honest with yourself. This allows you to improve and also shows you where your limits are. After that is established you are set to do great things in the world, up to your limits. Maybe you have low IQ and work a simple job. For you Magnanimity would mean showing up for work on time and doing excellent work. Humility is needed to see this.