How do I manually add a review score that is not an average of the multi criteria?

I never recommend editing the parent theme files but if you really want to add your own review score manually without it being an average of the multiple ratings you've added do the following:

Open lib/inc/ratings.php and find:

$GLOBALS['ghostpool_total_score'] = number_format( $GLOBALS['ghostpool_site_rating_sum'] / count( $GLOBALS['ghostpool_site_rating'] ), 1 ) + 0;

Replace with:

$GLOBALS['ghostpool_total_score'] = $GLOBALS['ghostpool_site_rating'][0];

Find:

$GLOBALS['ghostpool_total_score'] = number_format( $GLOBALS['ghostpool_site_rating_sum'] / count( $GLOBALS['ghostpool_site_rating'] ), 1 ) + 0;

Replace with:

$GLOBALS['ghostpool_total_score'] = number_format( $GLOBALS['ghostpool_site_rating'][0], 1 ) + 0;

Open lib/sections/review-results.php and find:

$gp_site_ratings = $GLOBALS['ghostpool_site_rating'];

Replace with:

$gp_site_ratings = array_slice( $GLOBALS['ghostpool_site_rating'], 1 );

Now the first rating will only be used for the main score and the first criteria will not be used.