If you have an offshore account you have some wealth. You won't be putting a large % of your cash into bitcoin. Let's say you'd be ok with 3% of your offshore wealth in bitcoin.
Then, assume that only 2% of offshore accounts would actually do so. 2% is a reasonable adoption level.
So now you have 3% of 2% of offshore wealth in bitcoin, which is 0.06% of that 30 trillion dollars. That would be about $168k/BTC.
You can revise what you feel is more realistic, but your equation should account for (level of adoption %) x (% allocation in bitcoin given adoption).
The reason they would want to allocate their offshore reserves in bitcoin is so they DONT have to keep them offshore.