You can add more complex tax on your forms by using custom pricing rules to modify the form total depending on the answers to certain questions.
Setup
Add some questions to your form which will trigger the custom pricing rule. For this example, we will charge tax depending on the "Billing Country" selected.
Go to Configure → Payments → Custom Pricing rules and click "Add a rule" to add a new custom pricing rule.
Select the question you will use to trigger the custom pricing rule and create a rule to modify the total as required.
For example, we might add a 10% tax if the billing country is Australia, where 100% (original price) + 10% (tax) equals 110%, or 1.1 in decimal form, which is what we multiply the form's total by in the rule.
Add any other required pricing rules.
For example, we might want to add a 20% tax if the billing country is instead the United Kingdom.
Label
You can also set a label to describe the custom pricing rule in the checkout.
You may only use a single label for adjustments made by all rules combined.
Gotchas
Custom pricing rules are applied in order so if two rules are in effect at the same time the lower one will affect the rules above it.