OTM Calculate costs based on loading meter

My Shipment costs should be calculated based on loading meter. I can't find a suitable way of representing this in the rate record. My idea was to define a Flex Field on the shipment with the logic to calculate loading meters and then select this field in the rate record.

Does anyone know an alternative solution how else I can realize this?

The loading meter is calculated as follows:
Loading meter = length of the Ship Unit × width of the Ship Unit/ width of the Equipment