In my world, there used to be very powerful and influential wizard that maintains the balance in the world. Some kind of Pope figure. But unfortunately, he cannot escape death from old age. He dies, and the balance was broken.
The world becomes divided and several kingdoms have arisen. Now a kingdom further upsets this balance of power and started to levy taxes from all of their townspeople.
The concept of magic is common in this world, just as common as learning to solve a rubiks cube. Not everyone can use strong magical spells but anyone can learn to do so. (But difficult. Just like a student can become an engineer, or drop out of school completely)
As you imagine, a lot of people will be using magic in their everyday lives. For convenience, business, hunting, etc.
When using magic in my world, the human body emits an invisible aura that determines how powerful a magic has been used. For instance, casting a minor healing spell produces little amount of this aura, and casting stronger, destructive spells emits more. To put it simply, magic power has "tiers".
Now the government has a way of measuring its quantity and quality. I won't go to the specifics but let's just say the taxation distribution is assigned per household regardless the number of magic users in that family. Just for simplicity sake, let's rule out possible inconsistencies in calculating magic like "What if a friend used magic in another household?" and the likes.
Just to be clear, the tax coming from magic adds to the normal tax.
How would this work out differently than any other taxation system in history?