I read that the highest viewability rates of AdSense ads are obtained by placing them "near the bottom of the screen (right above the fold)"
[support.google.com ]

I am trying to find an agreed method to determine this 'fold' of a web page (the distance in pixels from the top). I'm aware that it depends on the screen dimensions and resolution, but I wondered if (e.g.) by using the 'toggle device' function in Chrome with a popular device (iPhone 12) could be ok.

Even more, does anybody use PHP ($_SESSION['screen_width'], $_SESSION['screen_height']) to calculate the fold for each visitor?