ok I figured it out on my own and though maybe some others would like to know how.
You dont need to edit anything in your formmail.php file to create or link to your own redirect page; rather than using the one the formmail file generates for you.
Open up your form page and add this "input type" line, which is listed in red, after your form actions.
<form action="formmail.php" method="post">
<input type="hidden" name="recipients" value="firstname.lastname@example.org">
<input type="hidden" name="subject" value="Form Details">
<input type="hidden" name="good_url" value="redirect.html">
name="good_url" is the variable name given to your redirect page by the formmail.php file
value="redirect.html" is the name of the redirect.html file you created
this is assuming your redirect file is the same folder as your form page - otherwise make the necessary changes to the url