In a linear Fresnel plant adopting slightly curved primary mirrors (cylindrical or parabolic), a significant gain in the collected radiation can be achieved using primary mirrors with different focal lengths, dependent on the position of the mirror with respect to the receiver. This work introduces a universal function that provides the optimal focal length of a mirror, given only the mirror’s position relative to the receiver and the latitude, for a NS-oriented collector with a flat horizontal effective target. In a solar plant with the focal lengths defined by this function, the efficiency gain with respect to a solar field adopting identical mirrors is estimated in the range 1.5–6%, with the gain increasing if mirror imperfections or tracking errors are present: this means that the regulation of the focal lengths is especially useful in containing the loss of efficiency due to defects. The given rule is tolerant to errors in the focal length regulation (up to 10%). The function can be used as a reference for future projects, or as a starting point for more refined optimizations.