1. Get the ID of the person I want to retweet
   
2. Get all of My Followers
3. Get the people that my followers follow
4. See how many of them follow the person I want to retweet.

This did work but it was horribly slow. Also, the twitter API is limited to 100 requests per hour. If you have over 100 followers, you won't get anywhere.

However, on closer reading of the API - and a little tiramisu - I had the answer.

1. Get my followers
2. Get the retweetee's followers
3. See how many from (1) occur in (2)

This can be accomplished in a few lines of PHP.

To get the first 5,000 followers for any user - use this URL

$myfollowers_url = "http://twitter.com/followers/ids.json?screen_name=" . $my_name . "&page;=1";

Then use curl to grab the data -

$curl_handle=curl_init();curl_setopt($curl_handle,CURLOPT_RETURNTRANSFER,1);
curl_setopt($curl_handle,CURLOPT_URL,$myfollowers_url);
$myfollowers_json = curl_exec($curl_handle);
curl_close($curl_handle);

You can turn the resulting JSON into an array thus:

$myfollowers_array = explode("," , $myfollowers_string);

You do the same for the person you wish to retweet.
PHP provides the method array_intersect() to see how many elements from array1 occur in array2.

$samefollowers_count = count(array_intersect($myfollowers_array,$theirfollowers_array));

From there, the maths is trivial.

$myfollowers_count = count($myfollowers_array);
$theirfollowers_count = count($theirfollowers_array);
$ratio = $samefollowers_count / $myfollowers_count;

Hey presto! Done. You can view the beta at https://shkspr.mobi/twitter/retweet.php