This article examines the scientific foundations of the methodology for determining the value added tax base on a margin basis. It analyses the substance of the margin approach introduced in Uzbekistan from 2026, its legal and methodological foundations, as well as the main practical problems and ways to address them. The study proposes the necessary conditions for the effective application of the margin-based tax base, an algorithm for VAT calculation, and a five-criteria filter for extending this approach to other segments. It also discusses the experience of the European Union and the United Kingdom, focusing on fiscal neutrality, documentation discipline, and control indicators in the application of the margin scheme. The article develops conclusions and proposals aimed at the scientifically grounded improvement of the margin-based VAT base methodology in Uzbekistan