How to determine mouse position in zoomed image

public Form1()
        {

            InitializeComponent();
            img = Image.FromFile( imagefilename );
            zoom = ((float)pictureBox.Width / (float)img.Width) * (img.HorizontalResolution / g.DpiX);
        }

                
        protected override void OnMouseWheel(MouseEventArgs e)
        {
            float oldzoom = zoom;

            if (e.Delta > 0)
            {
                zoom += 0.1F;
            }

            else if (e.Delta < 0)
            {
                zoom = Math.Max(zoom - 0.1F, 0.11F);
            }

                        
                        
             // ... some code



             // handle selection box
        if (rubberBandRect.Size.Width != 0) {
        float ppd = img.HorizontalResolution / g.DpiX;


        float a420x = ((float) rubberBandRect.X - oldImgX * oldzoom ) / oldzoom * ppd;
        float a420y = ((float) rubberBandRect.Y - oldImgY * oldzoom ) / oldzoom * ppd;

                                
        float a3x = a420x / ppd * (zoom );
        float a3y = a420y / ppd * (zoom );

        float a6x = imgx * zoom;
        float a6y = imgy * zoom;
        float npx = a6x + a3x;
        float npy = a6y + a3y;

        rubberBandRect.X = (int) npx;
        rubberBandRect.Y = (int) npy;

        float oldWidth  = rubberBandRect.Width  / oldzoom * ppd;
        float oldHeight = rubberBandRect.Height / oldzoom * ppd;
                                
        rubberBandRect.Width  = (int) ((float) oldWidth  / ppd * zoom);
        rubberBandRect.Height = (int) ((float) oldHeight / ppd * zoom);
}



        private void pictureBox_Paint(object sender, PaintEventArgs e)
        {
            GraphicsState save = e.Graphics.Save();
            e.Graphics.InterpolationMode = InterpolationMode.HighQualityBicubic;
            e.Graphics.ScaleTransform(zoom, zoom);
            e.Graphics.Restore( save );                 
        }