I have some data which looks like this:
I get a number in the -6/+4 range and I need to express it with percent. If I get 4 of course the result is 100%, If I get -6 is 0%, etc.
Is there a quick formula I can use to obtain the percent value of, say, -3.25?
For $x \in [-6, 4]$ the function $$p(x) = \frac{x+6}{10}$$ is what you are looking for. You should note however that this gives numbers between $0$ and $1$. If you want the actual percentages, just use the function $$q(x) = p(x) * 100 = 10 (x + 6)$$ instead.
In general for an interval $[a, b]$ you can use the formulas $$p(x) = \frac {x-a} {b-a}$$ and $$q(x) = 100 * p (x) = 100 * \frac {x-a} {b-a}$$