Hmm, missed this codex page… To solve this the number should be passed as a decimal as a string rather than an int;
20 -> '19.1'
adjusted code;
add_menu_page( 'Settings', 'Settings', 'manage_options', 'plugin_name', 'plugin_name_options_page', content_url( '/img/icon.png', __FILE__), '19.1' );