This paper gives an analytical description of the stationary regime of a storage-ring free-electron laser in the presence of the maximum detuning (compatible with the laser onset) between the laser pulse and the electron beam when they pass and interact in the optical cavity. In this condition, the conservation of the first moments of the laser intensity distribution allows one to express the peak gain of the light amplification process and the maximum detuning as a function of system parameters that are directly measurable. These theoretical results are compared with experiments performed on the Super-ACO free-electron laser.