Capacity: Max mana. The amount of "magickz" the body can store without complications. It's not a hard cap, but results in complications like Nicole has, like the body forcefully shutting down and releasing excess mana while unconscious.
Potential: Max "burst" mana. The amount of "magickz" the body can put into a single spell. If one had low "potential", they'd need to train it up before they can cast magickz which require lots of "magickz."
I think the problem lies in the explanation. I think Nicole's potential is bigger than her capacity. Her body tries to store mana to match her potential, exceeding her capacity in the process. I don't have explicit suggestions on how to get this across in the manga panels.
Potential: Max "burst" mana. The amount of "magickz" the body can put into a single spell. If one had low "potential", they'd need to train it up before they can cast magickz which require lots of "magickz."
I think the problem lies in the explanation. I think Nicole's potential is bigger than her capacity. Her body tries to store mana to match her potential, exceeding her capacity in the process. I don't have explicit suggestions on how to get this across in the manga panels.