Again, I would suggest not doing that because it opens up the opportunity to introduce data inconsistencies. As a general rule of database design you should never store a value that can be calculated from other values on the same row.
However if you want to it could be done with php on the insert or update page Server Behavior setting the value from the submitted form by binding it to something like:
<?php echo(round(($_POST['field1'] + _POST['field2'] + _POST['field3'] + _POST['field4'] + _POST['field5'])/5,2)); ?>
Did this help? Tips are appreciated...
