Solution A: When the chosen way of transport isn't over land, OR when the locations are very far apart; e.g. China to Europe, enter the distance manually.
Use https://app.searoutes.com/routing or http://ports.com/sea-route/ to calculate the distances over sea.
* 1nm is equal to 1.852 KM
Solution B: Use a worst case scenario of container ship and 20 000 km's between from and to locations. By doing this, you will not greenwash since a scenario that will have higher emissions is very unlikely.
We are working on calculating distances automatically. This may not be fully in place, or the selected combination of transport method and from and to locations can't be combined.