You're the one having things the wrong way round, specifically the wrong divisor. The formula is idiotic for other reasons, i.e. it has nothing to do with "tariffs" or "reciprocal", it's like asking "how much are eggs at Aldi today" and USTR provides a formula: number_four_letter_words_in_your_posts / total_number_of_words_in_your_posts.

However in the name of science, could you please try to plug in numbers for Vietnam into the formula and show us how it equals your magical reciprocal number of "2".

Here is the source data: https://www.census.gov/foreign-trade/balance/c5520.html