What line are you getting the error at. If you want to convert a string to a Number you generally have to use parseFloat( someString ).
percent doesn't look like it is set to a value. It will default to NaN and if you add anything to NaN you get NaN.
protected function init():void
var num1:Number = 5;
var sum:Number = num1 + num2;
You might want to initialize percent to zero.