Making the tax rules based on the dynamic data directly is not possible within eCart. There are other ways you can do this though. If you have session variables that you set based on the selection of the user you can use these session variables in the rules.
The idea is that you have the user select the county then you would use this value to filter a recordset. The values that you get in the rs would then be stored in session variables. You would then use these session variables as the trigger and amount of the tax for the selected county.
To start with on this you will need to ensure that you are storing the tax rate in the db along with the county. You should store this tax rate as a decimal value, so it will be like 0.05 for 5%.
This is the general high level overview, if you would like to go this route let me know what questions you have about any part of this and I will be glad to help you with what I can.