Hello there, Phil.
Let's make sure that you entered and categorized all of your business transactions to have an accurate sales tax. Also, you can review the information entered in your Tax Profile settings. Please refer to this page for more details: How we calculate estimated tax payments.
I'll be adding articles that will also help you in handling QuickBooks Self-Employed and how to categorize transactions:
Keep me posted if you have any other question in mind about QuickBooks Self-Employed. Take care!