Find and display lowest value from one field in a node type
It is not clear from your question if your 'aggregated uses' and 'available uses' are CCK fields already. If they are, I hope this helps... You can access each of the fields available $a and aggregated $b with this code: 

$items = field_get_items('node', $node, 'field_YOUR_FIELDNAME');
$first_item = array_shift($items);
$YOUR_VAR = $first_item['value'];   


then deduct $a-$b=$c and store $c in another CCK field using the node_load and node_save:

 $node = node_load($nid = YOUR_NID);
 $node-&gt;field_YOUR_FIELDNAME[$node-&gt;language] = array(
    0 =&gt; array('value' =&gt; $c)
 );


once $c field is stored in your node, you will be able to access and display it from Views.