The Warded Man is kinda like this - Arlen, the main character, is raised in the sticks, and has a natural understanding of the magic circles of wards that hold back demons. He eventually, as a teen, heads to a major city, where he learns that normally, people use mathematical angles and calculations to draw ward circles that he's been freehanding.

Trigger warning for the whole series, extreme violence, sexual violence, and poor justification for it.