How to change grand_total value from Sales Order?

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1

I have some extra calculation for grand total in the sales order. But I can’t adjust my calculated results with original grand_total value.

class BkSalesOrder(SalesOrder):
    def validate(self):
        self.grand_total = 169.9
        self._add_payments_record()
        super(SalesOrder, self).validate()

    def _add_payments_record(self):
        list_of_payments_schecule: list = []
        self.agreement_period = int(self.agreement_period)
        rental_price = ItemRentalPrice(float(self.rounded_total), int(self.agreement_period))
        monthly_rental_amount: float = rental_price.calculate()

        #list_of_payments_schecule.append(PaymentSchedule('Transportation Fee', 10.00, self.name + '-' + 'Payment 1'))
        list_of_payments_schecule.append(PaymentSchedule('Payment 1', monthly_rental_amount, self.name + '-' + 'Payment 2'))

        for i in range(1, self.agreement_period):
            list_of_payments_schecule.append(PaymentSchedule('Payment {0}'.format(i), monthly_rental_amount, self.name + '-' + 'Payment {0}'.format(i + 1)))
        
        print(sum([obj.monthly_rental_amount for obj in list_of_payments_schecule]))
        for index, schedule in enumerate(list_of_payments_schecule):
            payment_schedule = self.append('payment_schedule', {})
            payment_schedule.number_of_payment = schedule.no_of_payment
            payment_schedule.payment_amount = schedule.monthly_rental_amount
            payment_schedule.payment_id = schedule.payment_id
            payment_schedule.due_date = self._get_next_due_date(datetime.now().date(), index)
  
    def _get_next_due_date(self, date, i):
        add_month_to_date = add_months(date, i)
        if isinstance(add_month_to_date, datetime):
            return add_month_to_date.date()
        else:
            return add_month_to_date

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