A general and accurate formula for evaluating the single-sided beamwidth in one-dimensional unidirectional leaky-wave antennas has been recently proposed. The half-power beamwidth is then estimated by simply doubling the value of the single-sided beamwidth. However, this approximation may lead to inaccurate results as the beam approaches endfire; at that point, the left- and the right-sided beamwidth may considerably differ one another. This conference paper proposes and numerically validates a more general formula to solve this issue.