USGTI - Provide a US Sourced Gross Transport Income calculation

From 2015 onwards, the United States have imposed a 4% tax on all foreign corporation’s US source gross transportation income. The income is defined as 50% of the non-US corporation’s shipping income pertaining to its voyages to, or from, the United States.

This feature requests suggest a standard calculation of the US Sourced Gross Transport Income, which can be used for the US Tax declaration.