Calculate percentage on negative to positive scale Scale 1:
-5  -4  -3  -2  -1  0   1   2   3   4   5
Scale 2:
1   2   3   4   5   6   7   8   9   10  11

How do I calculate the percentage of value X in Scale 1 when 0 is a valid value on the scale?
Ex.
3 in Scale 2 would be 3/11*100 = 27.3%
So whats the function to get -3 in Scale 1 to be 27.3% of its scales total steps?
 A: Scale $1$ runs from $-5$ to $5$ in steps of $1$ unit, so it has $11$ values. $-3$ is the third value. Thus, $3$ of the $11$ values are less than or equal to $-3$, so $\frac3{11}$ of the values are less than or equal to $-3$. If you convert this fraction to a percentage, you get $27.3$% when you round to one decimal place.
More generally, if a scale runs from $a$ to $b$ in steps of size $d$, it has $\frac{b-a}d+1$ values. If $v$ is one of the values, there are $\frac{v-a}d$ values smaller than $v$, so $v$ is the $\left(\frac{v-a}d+1\right)$-st value. 

In Scale $1$, for instance, $a=-5$, $v=-3$, and $d=1$, so $-3$ is the $\left(\frac{-3-(-5)}1+1\right)$-st value or, after you do the arithmetic, the $3$-rd value. (Of course in this case we don’t need the formula: we can easily see that $-3$ is the $3$-rd value.) 

Thus, we have altogether $\frac{b-a}d+1$ values, and $\frac{v-a}d+1$ of them are less than or equal to $v$, so the fraction of them that are less than or equal to $v$ is
$$\frac{\frac{v-a}d+1}{\frac{b-a}d+1}=\frac{v-a+d}{b-a+d}\;,$$
and you can convert this to a percentage simply by multiplying by $100$. In the example with $a=-5$, $b=5$, $d=1$, and $v=-3$, this formula yields the fraction
$$\frac{-3-(-5)+1}{5-(-5)+1}=\frac3{11}\;,$$
as we knew it should.
