How to Calculate Similarity Percentage with Levenshtein function

Hi,
I'm trying to obtain the similarity percentage for 2 Strings when using
the Levenshtein function but the percentages doesn't seem to match the
percentage
returned by similar_text($str1,$str2,$match_percent);

Below is what I use to calculate the similarity percentage

$dist = levenshtein($str1,$str2);
$str1len = strlen($str1);
$str2len = strlen($str2);
if($str1len > $str2len)
{
$pct = ($str1len - $dist)/$str1len;
}
else
{
$pct = ($str2len - $dist)/$str2len;
}
$pct = $pct*100;

What am I missing from the calculation?

Thanks.