Process the from by myself

Currently the only options for hooking into the Webform processing are:
- hook_form_alter() to add a #submit or #validate handler that can use form_set_value() to set a value to be saved
- The additional processing and validation fields located under "Webform advanced settings" when editing the Webform. You can set the values of the form into the $form_values['submitted'] array and they will be saved into the database.

Quicksketch, I'm (using 5.x-2.1.2) trying to do exactly what you recommend in your second bullet point and get errors:

code sample from "Additional Processing":

//  other code above here crunching numbers
//  based on values entered in webform fields
$t = $t1 + 40 - $t2;
$r = $r1 + 40 - $r2;
$form_values['submitted']['t'] = $t;  // field is called t
$form_values['submitted']['r'] = $r;  // field is called r

results in:
user warning: Duplicate entry '6-0-0' for key 1 query: INSERT INTO webform_submitted_data (nid, sid, cid, no, data) VALUES (2, 6, 0, 0, '6') in /srv/www/drupal-5.1/includes/database.mysql.inc on line 172.

The the nid and the sid are correct, but the cid is always zero, and I don't know what no is. Each time the form is submitted, one entry does wind up inserted into webform_submitted_data with a cid of zero. Of course that works once, then results in a duplicate key error for subsequent attempts.

The field I am referencing has been defined as a hidden field (tho I did get the same result trying it with a textfield). This is a multi-page form, and all the hidden fields are on the last page. I have tried setting default values in the field definition and that makes no difference. Run out of things to try. Any thoughts? Thanks so much.

and of course the $form_values['submitted'] array is indexed by cid, not by field name, hence the missing cid. sometimes all it takes is posting it to make the answer totally clear. :)

//  other code above here crunching numbers
//  based on values entered in webform fields
$t = $t1 + 40 - $t2;
$r = $r1 + 40 - $r2;
$form_values['submitted'][43] = $t;  // field is called t
$form_values['submitted'][44] = $r;  // field is called r

works like a charm.

Fantastic juberto :)