Math help calculate percentage

Hello,

As the tittle says i need some math help to calculate percentage.
This is my script
$ps is the percentage. What the script does is: It has a specific number and every sec that number gets lowered.
I have managed getting it percentage and count it but my problem is that is the wrong way.
It has to rise from 0% to 100%. Now it is the oposite. It is 100% and as the number gets lowered it gets too.

Any kind of help appretiated