Converting marks to % when the lowest possible mark is not zero

Question

A multiple choice questionnaire has $10$ questions. The marking scheme is $+5$ for correct answer, $−1$ for wrong answer.... The best mark would be $50$ and the worst mark $-10$. How can I convert the mark to percentage?

Answer 1

If you want a percentage of possible points, compute $$ g = \frac{r + 10}{60}, $$ where $r$ denotes the raw grade. Note that $0 \le g \le 1$.

You could alternatively compute the percentile of $r$ in the overall sample.