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...