Quick way to express percent from negative interval 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
$$q(x) = 100 * p (x) = 100 * \frac {x-a} {b-a}$$
