calculations that you define on the calcualtions tab of eCart are performed on a line item bassis, the grand total cannot be calculated on a line item bassis.
if you want to store the grand total *0.15966385 in a session value use:
$_SESSION['sessionName'] = WA_eCart_DisplayMoney($eCart_en, $eCart_en->GrandTotal()*0.15966385);
where eCart_en is the name of your eCart object.