  The normalization constant of the lateral distribution function (LDF) of an
extensive air shower is a monotonous (almost linear) increasing function of the
energy of the primary. Therefore, the interpolated signal at some fixed
distance from the core can be calibrated to estimate the energy of the shower.
There is, somehow surprisingly, a reconstructed optimal distance, r_{opt}, at
which the effects on the inferred signal, S(r_{opt}), of the uncertainties on
true core location, LDF functional form and shower-to-shower fluctuations are
minimized. We calculate the value of r_{opt} as a function of surface detector
separation, energy and zenith angle and we demonstrate the advantage of using
the r_{opt} value of each individual shower instead of a same fixed distance
for every shower, specially in dealing with events with saturated stations. The
effects on the determined spectrum are also shown.
