How to Set Location of Mdichild at center of MdiParent

I tried to set location of a modeless Form of a MdiChild (ShippingDialog ) at the center of its MdiParent (CreateOrderView.MdiParent) with below code.

The horizontal (X) value of MdiChild is only little bit off while vertical (Y) value of MdiChild is totally off. I expected 5.4 centimeters while I got 9.7 centimeters between Top of the MdiChild and MdiParent. Any idea what I am missing here?

Code:

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if(ShippingDialog != null)
            {             
                // set MdiParent and location for new dialog only
                if (newDialog == true)
                {
                    // Set EquipmentShippingAddress inside of MdiParent (CallManager)
                    ShippingDialog.MdiParent = CreateOrderView.MdiParent;

                    // Manually set EquipmentShippingAddress at center of MdiParent (CallManager)
                    ShippingDialog.StartPosition = FormStartPosition.Manual;
                    ShippingDialog.Location = new System.Drawing.Point(ShippingDialog.MdiParent.Location.X + (ShippingDialog.MdiParent.Width - ShippingDialog.Width) / 2, ShippingDialog.MdiParent.Location.Y + (ShippingDialog.MdiParent.Height - ShippingDialog.Height) / 2);
                }

                ShippingDialog.Show();             
                ShippingDialog.BringToFront();
            }

---

thanks in advance,
Xiaoming