The Tax calculation is only done once based on the original customer's location. The most specific Tax Zone will be used to calculate the tax at the time of Quote generation. Also, only one Tax Zone will be used per each Quote calculation.
To create a Tax Zone simply go to Settings > Taxes > + New Tax Zone:
You'll then be presented with the following screen to enter the appropriate Tax(es):
If you've set the Region as optional within the Required Fields, you'll need to ensure the Tax is set at the Country level