Hi there and welcome to the Seller Community!

You can accomplish the percentage tax by changing your preferences within the Point of Sale app. 

Visit the Settings section of the app, tap on "Taxes" and then tap on the tax you've created. 

Make sure the "Applicable Items" is set to all items, and the "Item Pricing" is set to add tax to price. 

This way, your sales tax will be applied to the entire sale. 

Was this what you were looking to do? Let me know if you need more help.