I suppose you're referring to how QuickBooks Self-Employed (QBSE) gives you an estimate for federal income and self-employment taxes. If that's the case, it is calculated by applying income tax rates to your total self-employment taxable profit from your business plus any personal income you enter from outside your company. Here's how you can view your tax estimate:
Please know that you can always get back to this post if you have other questions about estimated quarterly taxes. I'm always here to help. Have a good one.