Hi @bradwiatr
WP contains some handy functions to retrieve urls / paths to the WP installation. Something like this should work a treat:
include_once( get_template_directory() . '/lib/acf/acf.php' );
add_filter('acf/settings/dir', function( $dir ){
return get_template_directory_uri() . '/lib/acf/';
});
add_filter('acf/settings/path', function( $path ){
return get_template_directory() . '/lib/acf/';
});