I'm not particularly in favour of tracking language level scientifically, because it's a very subjective level. Judging someone as B2 or C1 means nothing in real life, just that they passed one test one day.
Having said that, if you equate C2 to "perfect" native speaking level (which by the way, virtually NO native speakers achieve in their lifetimes...), then you can get scientific by measuring distance to 100%, where distance is the number of objective errors. You would need to track 4 distinct areas: listening, speaking, writing, and reading:
* Listening: use a variety of sources (tv programs, radio, audiobooks, informal youtube videos or interviews). Listen at normal speed for the first time, and truthfully measure how many words per hour you did not understand. This is an objective measure that you can track. You might want to track different media separately (rap music is 1000x harder than a interview with a politician...).
* Writing: use a variety of styles, write without a dictionary, and get them corrected. Can you write formal and informal styles (say, a news report vs a love letter vs a movie review), and get them corrected by a native? Then you can track number of errors objectively.
You get the idea, similar for speaking (you'll need a teacher or very good native speaker to pick up errors and count them up while you speak at natural speed with them!) and reading (easiest one to do).
This will absolutely not judge if you're native level in terms of natural conversation in a noisy place, and it will not tell you if you are C2 or not (C2 is just decided on the basis of your knowledge of one exam on one day, so if the qualification is important to you, then this scientific method will not do much to progress you towards that goal), but if you absolutely need a chart with numbers, that's how I would do it.