I keep thinking about the question, and in a lot of ways BTN & WTN are pretty specific to Gurps as they are derived from the Traveller Conversion to Gurps term of the hodge-podge that was Traveller.
Also those stats were derived thematically from material in T4's Pocket Empires.
You might be better off just rewriting using the mechanics present into MgT's originated data set.
I went thinking about these numbers if terms of ship traffic and gross economic factors then to got with a base Volume of trade based off of the world's Population (Gross not statistic) in physical volume then modified by the said worlds trade classifications, Then by the gravity model of trade. Though a healthy dose of The Central Places Theorem is laid over the top of this, but that is natural as I am a geographer by trade.
The general mass rule I start with is Gross Population in Cubic Meters is the physical volume of trade with worlds with Resource, Non Ag and Ag having higher shipping volumes as they tend to ship bulk goods. Then I use the old Tech Level vs Starport matrix for credit conversion to generate a generalist valuation of the shipped mass.
This is an area that has engendered a lot of conversation over the years, and circles around a number of theories and rules mechanics that all interrelated, such that while a specific set of rules/terms might be under a copyright the expression of the underlying information isn't.