Would be good to know -
Who are the people coming up with these specific rates?
How are they modeling this system?
It's trade deficit divided by total exports divided by 2.
According to the BBC reporting, it appears that for each country he's charging 50% of the tariff they place on US goods coming into those countries, or a baseline of 10% for some countries like the UK.
(sorry, this is a BBC live news ticker link):
https://www.bbc.com/news/live/c1dr7vy39eet?post=asset%3A3a34...
And a picture of the chart they had showing the per-country tariffs:
https://www.bbc.com/news/live/c1dr7vy39eet?post=asset%3A7f78...
I am certain they didn't use any liberal math or science or history to arrive at these numbers. It's all just made up by incompetent yes men.