Only add the "add more" link when required

gistfile1.php

<?php

function ca_only_add_more_when_required( $default, $text, $options ) {
    $text = strip_tags( $text );
    if ( 'words' === $options['length_type'] ) {
        $excerpt_length = str_word_count( $text );
    } else {
        $excerpt_length = strlen( $text );
    }

    if ( $excerpt_length > $options['length'] ) return $default;

    return true;
}
add_filter( 'advanced_excerpt_disable_add_more', 'ca_only_add_more_when_required', 10, 3 );