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.
A lot of comments have already mentioned that. You can also calculate it by yourself.
For example, in 2024, the total value of US trade in goods with China was approximately $582.4 billion, comprising $143.5 billion in exports and $438.9 billion in imports. [0]
Who are the people coming up with these specific rates?
How are they modeling this system?