How to bound a circle inside an ellipse?

Question

The title for this post was quite hard to think of, so if you can think of a more descriptive title please tell me. Anyway, my problem is quite specific and requires some simple maths knowledge. I am writing a C# WinForms application which is a bit like the old 'xeyes' Linux application. It basically is a set of eyes which follow around your mouse cursor. This may sound easy at first, however can get rather complicated if you're a perfectionist like me :P. This is my code so far (only the paint method, that is called on an interval of 16).

int lx = 35;
int ly = 50;
int rx;
int ry;

int wx = Location.X + Width / 2;
int wy = Location.Y + Height / 2;

Rectangle bounds = Screen.FromControl(this).Bounds;

// Calculate X

float tempX = (mx - wx) / (float)(bounds.Width / 2);

// Calculate Y

float tempY = (my - wy) / (float)(bounds.Height / 2);

// Draw eyes

e.Graphics.FillEllipse(Brushes.LightGray, 10, 10, 70, 100);
e.Graphics.FillEllipse(Brushes.LightGray, 90, 10, 70, 100);

// Draw pupils (this only draws the left one)

e.Graphics.FillEllipse(Brushes.Black, lx += (int)(25 * tempX), ly += (int)(40 * tempY), 20, 20);

Now this does work at a basic level, however sometimes this can happen if the user puts the cursor at 0,0.

Now my question is how to fix this? What would the IF statement be to check where the mouse pointer is, and then reduce the pupil X depending on that?

Thanks.

Edit: This is where I get the mouse positions (my and mx):

private void timer_Tick(object sender, EventArgs e)
{
    mx = Cursor.Position.X;
    my = Cursor.Position.Y;

    Invalidate();
}

The timer is started in the eyes_Load event and the interval is 16.

Edit 2: Final solution: http://pastebin.com/fT5HfiQR