Designing cross-sections of prestressed beams based on Magnel's graphic method was first introduced in the mid-20th century. The graphic method is very useful for determining the initial prestressing force and its eccentricity action. This method is also used nowadays despite various computer programs accelerating the process of dimensioning prestressed structures. In the article, the author briefly outlined the idea of the Magnel method, and attempted to combine the classical method of adopting optimal dimensions for a cross-section of a post-tensioned concrete I-beam with the graphic method of Magnel to determine the prestressing force and its eccentricity. The purpose of this test is to simplify Magnel's graphic method and use analytical geometry in calculations to precisely determine the optimal prestressing force and its eccentricity. The prestressing forces and their eccentricity were calculated for beams with spans of 12.0 to 36.0 meters and the results are presented in diagrams.