In OpenERP HR Expense module we require some customization, I have added two fields one is advance taken from Company(x_adv) and due amount(x_dueamt).our requirement is calculate due amount (total_amount - x_adv). where and what should i write code.

for Example Employee A took advance from company for project X 500 INR But his total Expense is 1000 INR. Now we want to calculate due amount 500 INR automatically (x_dueamt is readonly).

Please Help us .... Thanks in advance ...