Well, this is either too simple of a question, or I just didn't understand it. If I did however understand it correctly, here's the way out:
Let's say you want to have a 1% chance of a falling heart. Since you're using a Number class for your chance
variable, and Math.random() returns a Number as well, you don't need any transformations.
Math.random() returns a Number (float) between 0 and 1, not including 1
so your code for 1% could look something like this:
var chance:Number = Math.random();
if (chance <= 0.01)
{
//enter code here
}
And yes, since you call less unneeded functions, it works faster. Math.random() gives a very precise number, far beyond 1/100, so it's possible to make a much less number for chance possibility, here's one value returned from Math.random():
Math.random(); // 0.9044877095147967