Greek Online Shop (OSCommerce) Totals Calculation

Greek Online Shop (OSCommerce) Need to change the way that the points redemption is calculated on Frontend and Backend and writed on "orders_total" Table on field "ot_subtotal_ex". Now the calculation of "ot_subtotal_ex" field is as bellow ot_subtotal_ex = Net product prices net I want the "ot_subtotal_ex" to be calculated as follow if a points redemption is present "ot_subtotal_ex" = Net product prices - ot_redemptions files that are used as I know (attached) admin/[login to view URL] ->lines 1858-1891 [login to view URL] ->find [login to view URL] admin/includes/functions/[login to view URL] ->lines 2211 necause it's a module maybe this to includes/modules/order_total/[login to view URL] or [login to view URL] see attached pictures after changes it must look like the pictures nr-2 End finaly apply the changes on admin/[login to view URL] (its a file from wich you can edit/change the order)