Information required for 'transit days' where these are variable

Due to sea freight shipping variability we have difficulties with our shipment routing.
We have a customer required date, which can be entered when the order is taken.
We can also estimate an 'average number of days' for the transit days to the Ship To destination address, which can be entered for the route as the default days.

The problem occurs when the freight is booked on a particular vessel and voyage, as depending on the voyage, there may be different port stops, and so the transit days for each shipment to this Ship To destination address can be different