I added an amount_fodec field in purchase.order to add specific tax of 1% on the amount_untaxed and then add it in the total amount. The problem is that the amount_total does not take into account amount_fodec. Here is my method:  

class PurchaseOrder(osv.osv):

_inherit = 'purchase.order'

 def _amount_all(self, cr, uid, ids, field_name, arg, context=None):

res = {}

cur_obj=self.pool.get('res.currency')

line_obj = self.pool['purchase.order.line']

for order in self.browse(cr, uid, ids, context=context):

res[order.id] = {

'amount_untaxed': 0.0,

'amount_tax': 0.0,

'amount_fodec':0.0,

'amount_total': 0.0,

}

val = val1 = val2=0.0

cur = order.pricelist_id.currency_id

for line in order.order_line:

val1 += line.price_subtotal

line_price = line_obj._calc_line_base_price(cr, uid, line,

context=context)

line_qty = line_obj._calc_line_quantity(cr, uid, line,

context=context)

for c in self.pool['account.tax'].compute_all(

cr, uid, line.taxes_id, line_price, line_qty,

line.product_id, order.partner_id)['taxes']:

val += c.get('amount', 0.0)

val2=val1/100

res[order.id]['amount_tax']=cur_obj.round(cr, uid, cur, val)

res[order.id]['amount_untaxed']=cur_obj.round(cr, uid, cur, val1)

res[order.id]['amount_fodec']=cur_obj.round(cr, uid, cur, val2)

res[order.id]['amount_total']=res[order.id]['amount_untaxed'] + res[order.id]['amount_tax']+res[order.id]['amount_fodec']

return res

any help please