您的问题是关于多屏幕显示器上的屏幕坐标，重点是：
[...]如果我决定把程序切换到我左边的显示器上怎么办？
您是否考虑过使用Screen类来获取显示指标？使用Jeremy的link for GetWindowRect()，您可以让应用按时间间隔轮询程序位置变化，确定每个应用所在的屏幕，并相应地调整坐标。

**组合框的屏幕 Package **

class ScreenItem
{
    public ScreenItem(Screen screen) => Screen = screen;
    public Screen Screen { get; }
    string _displayName = null;
    public string DisplayName
    {
        get => _displayName?? Screen.DeviceName.Substring(Screen.DeviceName.LastIndexOf('\\') + 1);
        set => _displayName = value;
    }
    public override string ToString()
    {
        List<string> builder= new List<string>();
        builder.Add($"{nameof(Screen.Primary)} : {Screen.Primary}");
        builder.Add($"{nameof(Screen.Bounds)} : {Screen.Bounds}");
        builder.Add($"{nameof(Screen.WorkingArea)} : {Screen.WorkingArea}");
        return 
            string.Join(Environment.NewLine, builder.ToArray()
            .Select(_ => $"\t{_}"))
            + Environment.NewLine;
    }
}

列举

public MainForm()
{
    InitializeComponent();
    ScreenItem primary = null;
    foreach (var screenItem in Screen.AllScreens.Select(_ => new ScreenItem(_)))
    {
        if(screenItem.Screen.Primary)
        {
            primary = screenItem;
        }
        richTextBox.SelectionColor = Color.Navy;
        richTextBox.AppendText($"{screenItem.DisplayName}{Environment.NewLine}");
        richTextBox.SelectionColor = ForeColor;
        richTextBox.AppendText($"{screenItem}{Environment.NewLine}");
    }
}