def get_relative_center_location(parent, width, height):
    """get relative a center location of parent window.

    Parameters
    ----------
    parent (tkinter): tkinter widget instance.
    width (int): a width of a child window.
    height (int): a height of a child window..

    Returns
    -------
    tuple: x, y location.
    """
    pwh, px, py = parent.winfo_geometry().split('+')
    px, py = int(px), int(py)
    pw, ph = [int(i) for i in pwh.split('x')]

    x = int(px + (pw - width) / 2)
    y = int(py + (ph - height) / 2)
    return x, y