Instantiate prefab within parameters?(Javascript)

I have this script that lets you spawn a prefab in a grid pattern whenever you click. How would I make it so that you can only spawn the prefab within certain parameters(between certain X and Y values)? Here is my script:

var Block:GameObject;
var squareSize : float = 1.0f;
function Update ()
    if (Input.GetMouseButtonDown(0)) 
        var mousePos : Vector3 = Input.mousePosition;
        mousePos.z = 12.48309;
        var objectPos : Vector3 = Camera.main.ScreenToWorldPoint(mousePos);

        objectPos.x = Mathf.Round(objectPos.x/squareSize) * squareSize;
        objectPos.y = Mathf.Round(objectPos.y/squareSize) * squareSize;
        objectPos.z = Mathf.Round(objectPos.z/squareSize) * squareSize;

        Instantiate(Block, objectPos, Quaternion.identity);

Thanks in advanced!