Allows you to specify the excerpt length per page rather than through a global setting in your functions.php.

wp-excerpts.php

<?php

/** In functions.php **/

/**
 * Customise Excerpt Length
 * @param int $new_length Length of excerpt
 * @param mixed $new_more String to output at the end of the excerpt, if false reverts to default
 * @return type
 */
function custom_excerpt($new_length = 20, $new_more = false) {
    add_filter('excerpt_length', function () use ($new_length) {
        return $new_length;
    }, 999);

    if($new_more) {
        add_filter('excerpt_more', function () use ($new_more) {
            return $new_more;
        });
    }
    
    $output = get_the_excerpt();
    $output = apply_filters('wptexturize', $output);
    $output = apply_filters('convert_chars', $output);
    $output = '<p>' . $output . '</p>';
    echo $output;
}

/**
 * Customise String at End of Excerpt
 * @param dunno $more 
 * @return string
 */
function new_excerpt_more($more) {
    global $post;
    return ' <a class="more_link" href="'. get_permalink($post->ID) . '">Read More</a>';
}
add_filter('excerpt_more', 'new_excerpt_more');

/** Template Example **/

custom_excerpt(20);