Maybe this will help.
I spent several hours trying to understand why I couldn't get the following code to work :
TaxCalculator::set_for_country("FR", 0.196, "TVA", "inclusive"); (in _config.php)
I looked for a solution in the forum without finding it.
And finally, I got it ! I hadn't filled in the "administrator's country" field, in the security tab of the cms.
Now that the cms knows I'm French, the VAT is OK on my checkout page...
EDIT : it works if you're working on wamp (local server), and if you're connected.