How does OD capacity and output work?

So I read that people's magic circuits are fixed in quality and quantity from the moment they are born. The amount of Od that a person has is determined by their circuits as that is what produces Od. According to fate, an average matured magus has 25 units. Rin has 500 units and shirou has 20-30 units (25 According to El-Melloi 2 novel). However, in the Case Files material it says that Waver(age 19) had 20 units while El-Melloi 2 (age 29 to 32) had 70 units. This means it increased with time/training. This does go along with what Archer told Shirou about how with a decade of training, he could cast unlimited blade works by himself. The Case Files material also says the cutoff for a full-fledged magus is 100 units which contradicts the previous statement (unless the average matured magus is not a full-fledged magus).

What I am basically asking is can Od/magic circuits quality be increased through training or is it fixed? For example, shirou had 25 units at 17 with atrophied magic circuits. If he had opened them properly when he had first training at 9, would he have more after 8 years of training?