It's my fault that I confused Rudi. What I want to buy is a Star Alliance RTW ticket to take advantage of the bargain first class fare and get a still bigger bargain by buying it based on what I'd pay in Germany for the ticket.
What Rudi is describing is simply comparing the price of segments based on the US price vs. the price in the country that you are flying out of.
I don't see why the final segment must be purchased in the US. Since that segment does not originate in the US, it can be purchased in the country that the flight will originate from.