Updating a field via custom code

I have a hidden field called "paymentdue".

Under setup, submit, I have the following custom code

<?php
  $paymentdue = 115;
  if ($form->data['skillsessions'] == 'Yes') $paymentdue=$paymentdue + 45; 
  $form->data['paymentdue'] = $paymentdue;
?>

When I submit the form, the debug info shows that "paymentdue" has the correct value.

After the custom code I then call the DB Save action.

The problem I have is that this field is not saved to the database

Some debug info is below:

Array
(
    [option] => com_chronoforms5
    [chronoform] => Basketball_Enrollment_7to8
    [event] => submit
    [studentname] => Fred
    [studentsurname] => Smith
    [address] => 1243  Nowhere
    [suburb] => There
    [postalcode] => 02443
    [dob] => 27-10-2014
    [schoolyear] => Year 8
    [playedbefore] => No
    [experiencelevel] => No experience
    [team] => 
    [league] => 
    [parentname] => Andre
    [parentemail] => email@school.nz
    [parenthomephone] => 021 1234 432
    [parentmobile] => 13 23 23
    [vday] => 
    [time] => 
    [understandconditions] => 1
    [permission] => 1
    [skillsessions] => Yes
    [registration] => 1
    [paymentdue] => 160
    [paymentmethod] => Cash
    [button50] => Submit
    [719430a5484b618542b562bbae65f4c77d4f4ed5] => 1
    [id] => 22
)

Array
(
    [10] => Array
        (
            [Check Honeypot] => Array
                (
                    [0] => Honeypot check passed.
                )

        )

    [14] => Array
        (
            [DB Save] => Array
                (
                    [Queries] => Array
                        (
                            [0] => INSERT INTO `jexp_chronoengine_CF_Basketball_Enrollment_7to8` (`studentname`, `studentsurname`, `address`, `suburb`, `postalcode`, `dob`, `schoolyear`, `playedbefore`, `experiencelevel`, `team`, `league`, `parentname`, `parentemail`, `parenthomephone`, `parentmobile`, `vday`, `time`, `understandconditions`, `permission`, `skillsessions`, `registration`, `paymentmethod`, `user_id`, `created`) values ('Fred', 'Smith', '1243  Nowhere', 'There', '02443', '27-10-2014', 'Year 8', 'No', 'No experience', '', '', 'Andre', 'email@school.nz', '021 1234 432', '13 23 23', '', '', '1', '1', 'Yes', '1', 'Cash', '0', '2014-10-22 13:52:35');
                        )

                )

        )

)

The database does have a "paymentdue" field

That was it ... thanks :-)