Good afternoon, 

in my country there is a sales scenario where rounding is for the benefit of the customer. 

 example: 

if it is 10.78 the total of the invoice, it is rounded to 10.70 and 0.08 goes to the rounding.  How could I do so that my self.residual is the difference between the total of the document and the new field created self.redondeo

try with: 

(self.residual = self.amount_total-self.redondeo)

 but the change is not executed, this line added it in def _computer_amount (self), I don't know if it's the right way. thank you.