How to rate an empty distance in WA

Summary:

We have the following WA: 2 shipments linked as follows:

Shipment 1: From A (pick) to B (drop)

Empty distance (less than 100 km)

Shipment 2: From C (pick) to A (drop)

We want to rate the empty distances when is grater than 20 km as 1.1€ per km, so if the empty distance is 30 km, the extra cost should be: (30-20)km*1.1€/km= 11€

How do we configure this in our rate records?

We don't have an NFR stop, the shipments are just as I explained before.

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Version (include the version you are using, if applicable):

26A