If I know values for 0% and 100%, how can I calculate the value at 90%? If I have 2 values, one representing 0% of a range (lets say 1.0), and another value representing 100% (lets say 20.0), how would I work out what value 90% would be? A formula would be handy if poss. Thanks.

So, I take this to mean you have an interval of values, $[a,b]$ where $a$ is the min and $b$ is the max and you wish to find a point $90\%$ of the way along. We can write $x\in[a,b]$ as $x=a+(b-a)t$ where $t\in[0,1]$. It turns out that the value of $t$ in this formula is the percentage. At $t=0$ we get $a$, at $t=1$ we get $b$, and at $t=0.9$ we get $0.9b+0.1a$.

Again, the general formula (with $t$ as the percentage) is:

$$x(t)=a+(b-a)t=a(1-t)+bt$$