The paper deals with the application of the theory of tax justice and notably the principle of solvency in respect of personal income tax in the Czech Republic. As concerns the criteria of the correct, i.e. objective distribution, the paper focuses on the assessment of fiscal ability in the individual categories of natural persons who carry on business and other self-employed persons. With all of these categories being subject to a single tax rate, the paper focuses on forming the tax base, particularly in terms of meeting the main postulates included in the victim theory. An integral part of the paper is the proposal for tax policy changes in respect of the amount of what is known as flat-rate costs in the context of monitoring the actual amount of natural persons' costs. The paper is one of the outcomes of the grant project no. 7739, taking place at the University of Finance and Administration: "Tax policy in the context of economic cycle, social and pension policies of the Czech Republic".