Hello AlisonCox1, thanks for reaching out to the Community.   The income tax figure is calculated based on the income/allowable expenses and also the information on the tax profile. We have included an article on setting up the tax profile here for more detail.

 The section where it has personal allowance amount if you have not used the amount, you would leave the figure as it is, if you have used the amount with PAYE income for example then you would amend the amount to zero. That plus the date you started Self Employment will impact the calculation.

If you are not exceeding the 20 percent and after you check the tax profile set up as mentioned above and you cannot see why the figure is calculating if you could reply with a screenshot of the tax profile and the figures calculating on the report. Or if you prefer you can DM/PM us at QuickBooks UK on any of the following: Facebook ,Twitter, Instagram. 

We'd be happy to help you with any further questions.