over-riding $content_width not working

Hi
I am creating a child theme based on 2016

I want to set new upload media images

add_action( 'after_setup_theme', 'azm_add_image_size' );
function azm_add_image_size() {
  add_image_size( 'helen', 936 );
  add_image_size( 'brian', 227 );
}

add_filter( 'image_size_names_choose', 'my_custom_sizes' );
function my_custom_sizes( $sizes ) {
    return array_merge( $sizes, array(
       'helen' => __( 'helen' ),
        'brian' =>__( 'brian')
    ) );
}

The small size, brian, works fine.

However the large size is creating the correct image in the uploads folder eg ‘bluehousefrontelevation-1-936×624.jpg’

however, in the backend the media library is showing helen-840×560 and it adds the wrong width/height sizes into the code.

I have tired suggestions at https://codex.wordpress.org/Content_Width and core ticket

Nothing seems to work.

The only way i can find to get round it is to edit the functions.php within 2016 – which obviously I don’t really want to do – here’s what i edit:

function twentysixteen_content_width() {
//  $GLOBALS['content_width'] = apply_filters( 'twentysixteen_content_width', 840 );
    $GLOBALS['content_width'] = apply_filters( 'twentysixteen_content_width', 2000 );
}
add_action( 'after_setup_theme', 'twentysixteen_content_width', 0 );

Is there are better way to change this (annoying) variable please?