Hi,

You can check how the computation works in sale order. You can see the total amount is computing from the sale.order.lines. Just see the below code and adjust accordingly to your case.

Check this to see how to write compute field in Odoo: How to Write Compute Field and its Function in Odoo12

In Sales,

amount_total = fields.Monetary(string='Total', compute='_amount_all', track_visibility='always')

Compute Function:

@api.depends('order_line.price_total')
def _amount_all(self):
    """
    Compute the total amounts of the SO.
    """
    for order in self:
        amount_untaxed = amount_tax = 0.0
        for line in order.order_line:
            amount_untaxed += line.price_subtotal
            # FORWARDPORT UP TO 10.0
            if order.company_id.tax_calculation_rounding_method == 'round_globally':
                price = line.price_unit * (1 - (line.discount or 0.0) / 100.0)
                taxes = line.tax_id.compute_all(price, line.order_id.currency_id, line.product_uom_qty, product=line.product_id, partner=order.partner_shipping_id)
                amount_tax += sum(t.get('amount', 0.0) for t in taxes.get('taxes', []))
            else:
                amount_tax += line.price_tax
        order.update({
            'amount_total': amount_untaxed + amount_tax,
        })

Thanks