  In this letter we reexamine the evaluation of \zone in some proposed tests of
relativistic gravitomagnetism with existing and proposed laser--ranged
LAGEOS--like satellites in the gravitational field of the Earth. A more
conservative and realistic approach is followed by using the diagonal part only
of the covariance matrix of the EGM96 Earth's gravity model up to degree l=20.
It turns out that, within this context and according to the present level of
knowledge of the terrestrial gravitational field, the best choice would be the
use of a recently proposed combination using the nodes \Omega of LAGEOS, LAGEOS
II and LARES and the perigees \omega of LAGEOS II and LARES. Indeed, it turns
out to be insensitive both to the even zonal harmonics of degree higher than
l=20 and to the correlation among them
