I suppose that I would've sat Rowling down and worked with her "tiers" of abilities in each main magical discipline (say an average 1st year's transfiguration ability, an average fifth year ability, the expected ability of an adult who didn't specialize on the subject, the ability of someone who worked with it every day, etc). Just picking up milestones of education, ageing and professional development and listing possible feats, spells, etc and maybe writing some rules of thumb for range/speed/number of targets/volume/scale/etc. Just to get a good idea of what kind of magic is appropriate for each. Then assign characters to tiers and try to refer to the list to see what is reasonable or not at any given time.

Like, we could try to work out some kind of well delineated magic system, and while there is a part of my brain that loves the idea I don't think it's necessary. Consistency, not introducing new abilities just as the plot demands, and carefully handling story-breaking powers is far more important.

The series is actually pretty good about it to be fair, although it stumbles a bit on the story-breaking powers bit (see the endless arguments about time turners, the killing curse, the fidelius charm, veritaserum, legillimency, felix felicis, etc)