Hi mates, I'm new to odoo and using odoo 13. I'm trying to calculate maximum delivery to be made to a customer. Made a custom field in res.partner where we can add a percentage and that percent of product can be sent in delivery. When I add the amount more than the max it perfectly raises the exception, but when I add the amount within the max and entered product quantity the percentage becomes  zero.
Here's my code:

@api.onchange('qty_done')
def max_quantity_done(self):
   all_customers = self.env['res.partner'].search([])
   for customer in all_customers:
      max_qty = self.product_uom_qty + (self.product_uom_qty*(customer.allowable_delivery_qty/100))
      if self.qty_done > max_qty:
         raise UserError(_('For this customer the excess allowed quantity is ' + str(customer.allowable_delivery_qty) +'% and Max allowed Done quantity is ' + str(max_qty)))

I am unable to understand why this is happening