The beam-squint phenomenon, which is observed when offset portions of parabolic reflector surfaces are illuminated by circularly polarised primary feeds, can be a critical factor in the design of antennas employing frequency-reuse techniques. The letter describes the derivation of a very simple formula which can be employed to predict the magnitude of the beam-squint effect to an accuracy of better than 0.01 of the antenna halfpower beamwidth. Verification of the formula has been obtained using both computed and experimental data.