Cross-diameter flow measurements can be used as a measure of permeability of a core to complement conventional axial permeability. Conformal mapping is used to derive appropriate two-dimensional equations to calculate cross-diameter permeability from the measured flow rate and differential pressure. In some cases, particularly in pre-test measurements for API RP 43 standard perforation flow test, the flow measurement is performed along only a portion of the core length. Axial flow components allow some fluid to penetrate beyond the test segment. This results in artificially high flow rates and leads to errors in calculating permeability. The extent of the error depends on test geometry, core orientation and permeability anisotropy. In this paper, we use a finite difference numerical model to calculate correction factors which can be used to obtain correct values of cross-diameter permeability using the two-dimensional analytical equation.