Thanks for posting in the Shopify Community, I’m happy to look into this with you.
I recommend ensuring that you’ve set up your taxes according to the steps in this help document. Overrides only need to be used when the default tax rates don’t apply to products that you sell. For example, certain types of children’s clothing might be exempt from tax or have a lower tax rate. If the default rates don’t apply, then you need to create an override. You can read more about this here.
If you’ve determined that you’ve set up your taxes and overrides correctly but the wrong percentage is still being calculated at checkout, I recommend contacting our live support team through the Help Center. From there we can take a closer look at your store and investigate the issue.