Is there are better way to change this (annoying) variable please?
The Twenty Sixteen theme provides you with the twentysixteen_content_width
filter, where the default is 840
.
To modify it you should be able to use this within your child theme’s functions.php
file:
add_filter( 'twentysixteen_content_width', function( $content_width )
{
// Override the default 840 content width
$content_width = 2000;
return $content_width;
} );