Saving default field values on node_insert
You could probably do this in hook_node_presave(), e.g.:

function MYMODULE_node_presave($node) {
  // Get a value made up of other submitted fields, e.g.
  $computed_val = $node-&gt;field_first[$node-&gt;language][0]['value'] + $node-&gt;field_second[$node-&gt;language][0]['value'];

  // Assign that value to the other field
  $node-&gt;field_computed_field[$node-&gt;language][0]['value'] =  $computed_val;
}