Cooperative clinical trial group protocols frequently require off-axis point dose calculations. The Radiological Physics Center uses the calculative technique developed by Hanson et al. [Med. Phys. 7, 145–146 (1980); 7, 147–150 (1980)] to verify these calculations. In order to correct for off-axis energy changes, this technique requires off-axis half-value layer data, HVL, as a function of off-axis ray angle for the specific beam. This paper presents a formulism based on HVL measurements on a limited number of therapy beams, which allows the calculation of an off-axis energy-correction factor for any clinical photon beam created by a linear accelerator using conventional flattening filters.