Constraining movement of movie clip?

1

Hi I am making a movie - in which each time it loads a moviclip appears in a different place. I am using the code

onClipevent(load) {
this._x = Math.random() *???;
this._y = Math.random() *???;

Where the questions are I don;t know what figures to put in,

I have covered the area I want it to only appear in and it reads as
w 386.0 x150.2
H 303.3 y 91.0

But i don;t know how to make these numbers fit in the above code, i would be really greateful if anyone could help,
lesley