Getting the position of a widget in a Flutter test?

How do you get the position of a Widget in a Flutter test? Ideally the center of the widget, but the top left coordinate would be fine too.

This is my test and attempt at a function that finds the location of a Widget on screen.

  testWidgets('Pin is in center of screen', (WidgetTester tester) async {
    await _setUp(tester); // Does setup
    final findPin = find.byKey('pin');
    final pinLocation = _getLocation(findPin.evaluate().first);
    print(ui.window.physicalSize);
    expect(pinLocation.dx, 1200.0);
    expect(pinLocation.dy, 900.0);
  });

Offset _getLocation(Element element) {
  return (element.findRenderObject() as RenderBox).localToGlobal(Offset.zero);
}

The printing for ui.window.physicalSize (where ui is the dart:ui package) tells me the size is Size(2400.0, 1800.0). However, when I set those midpoints in my test, my test fails, saying that the Size is actually 395.0 and 305.0. My widget is only 30.0 x 30.0 in size, and even 395 +/- 30 doesn't equate anything in the 1200 range.

In my test, I then wrapped my widget inside a Container of predefined width (400) and height (300), which is not how it looks in the real app, but for the purposes of testing it's much closer - giving a "center" of 195 and 155, when it should be 200 and 150.

The widget I'm testing (in the code, not with the extra Container in my testWidgets) is approximately:

return Center(
    child: Container(
      key: Key('pin'),
      height: 30.0, 
      width: 30.0, 
      color: Colors.pink,
   ),
);