Centering controls in UWP C#

I'm trying to center a textblock in my app window. I tried this code inside the textblock's Loaded event handler:

    private void textBlock1_Loaded(object sender, RoutedEventArgs e)
    {
        Debug.WriteLine("textBlock1_Loaded");
        double textBlockWidth = textBlock1.Width;
        double textBlockHeight = textBlock1.Height;
        double gridWidth = grid1.Width;
        double gridHeight = grid1.Height;
        double leftRightMargin_center = (gridWidth - textBlockWidth) / 2;
        double topBottomMargin_center = (gridHeight - textBlockHeight) / 2;
        double topMargin_needed = topBottomMargin_center - 25;
        double bottomMargin_needed = topBottomMargin_center + 25;
        double leftMargin_needed = leftRightMargin_center;
        double rightMargin_needed = leftRightMargin_center;
        textBlock1.Margin = new Thickness(leftMargin_needed, topMargin_needed, rightMargin_needed, leftMargin_needed);
    }

Here's the resulting app:

Am I doing something wrong in here? And yes, that's the only question I have in this thread.