Is it incorrect to say "your grade is calculated by $\text{(grade / out of)} \times 100\%$? Is it incorrect to say "your grade is calculated by $\text{(grade / out of)} \times 100\%$ $100\%$ is $1$ as a number. So if you got $9/10$ on a test, your grade would actually be $90\%$ But the way many introductory textbooks put it is that your grade is $(9/10)\times 100\%$, which is wrong since this is $0.90$. Should they instead write "your grade is calculated by $[\text{(grade / out of)} \times 100]\%$ When you want stuff expressed as a percentage, wouldn't it be more correct to put the **percentage symbol outside**?

If you view the % symbol as an abbreviation for $1/100$ then, for example, $$0.9 \times 100\% = 0.9 \times 100 \times 1/100 = 90 \times 1 / 100,$$ which is $90\%$ just as you would expect. This is the sense in which $100\%$ is $1$ as a number.

Because multiplication is associative, this means that it does not matter if the % sign is inside or outside the parentheses.