How can I validate length of text after form and data is loaded?

I have developed simple form that users will input data.  I've written validation scripts to show the user how many characters remain available.  The script works great so long as the form that is loaded is expecting new data entry.  Now I'd like to use that same form for users to edit previously entered data.  What do I need to change in this script so the length of data loaded is taken into account when the window is loaded?

My form has an input
<input type="text" name="fieldname" id="fieldname"       maxlength="20" placeholder="Field Name"
value="<?php echo $row['FieldName']; ?>">

and my form has a feedback message
<p style="text-align: right" id="feedback_fieldname"></p>

Here is my validation script:
<!--Inform User Remaining Length of input fields-->
<script>
window.onload = function() {

    var text_max_fieldname = 20;
    $('#feedback_fieldname').html(text_max_fieldname + ' characters remain ');

    $('#fieldname').keyup(function() {
        var text_length_fieldname = $('#fieldname').val().length;
        var text_remaining_fieldname = text_max_fieldname - text_length_fieldname;

        $('#feedback_fieldname').html(text_remaining_fieldname + ' characters remain ');
    });

}
</script>