I hope you don't mind if I add a few suggestions: the post author's posts and joindate (joindateline) are already available, so you don't need a query. You could use the template hook postbit_userinfo_right_after_posts so that you don't need to edit the template. So maybe something like this (using postbit_display_complete):
Code:
if (($days = round((TIMENOW-$post['joindateline']) / 86400)) <= 0)
{
$days = 1;
}
$ppd = round($post['posts'] / $days, 2);
$template_hook['postbit_userinfo_right_after_posts'] .= "<dt>Posts Per Day</dt><dd>$ppd</dd>";
There is a phrase that says "Posts Per Day" but it isn't loaded for the showthread page, so if you need phrases or would rather use one, you'd have to load the 'user' phrases, or just create a new phrase using the 'postbit' phrase type.