  We investigate stellar magnetic activity from the theoretical point of view,
by using stellar evolution models to calculate theoretical convective turnover
times ($\tau_{\rm c}$) and Rossby numbers (${\rm Ro}$) for pre-main-sequence
and main-sequence stars. The problem is that the canonical place where
$\tau_{\rm c}$ is usually determined (half a mixing length above the base of
the convective zone) fails for fully convective stars and there is no agreement
on this in the literature. Our calculations were performed with the ATON
stellar evolution code. We concentrated our analysis on fully and partially
convective stars motivated by recent observations of slowly rotating fully
convective stars, whose X-ray emissions correlate with their Rossby numbers in
the same way as in solar-like stars, suggesting that the presence of a
tachocline is not required for magnetic field generation. We investigate the
behaviour of $\tau_{\rm c}$ over the stellar radius for stars of different
masses and ages. As ${\rm Ro}$ depends on $\tau_{\rm c}$, which varies strongly
with the stellar radius, we use our theoretical results to determine a better
radial position at which to calculate it for fully convective stars. Using our
alternative locations, we fit a sample of 847 stars in the rotation-activity
diagram ($L_{\rm X}/L_{\rm bol}$ versus ${\rm Ro}$) with a two-part power-law
function. Our fit parameters are consistent with previous work, showing that
stars with ${\rm Ro}$$\leq$${\rm Ro_{sat}}$ are distributed around a saturation
level in $L_{\rm X}/L_{\rm bol}$ and, for stars with ${\rm Ro}$$>$${\rm
Ro_{sat}}$, $L_{\rm X}/L_{\rm bol}$ clearly decays with ${\rm Ro}$ with an
exponent of $-2.4\!\pm\!0.1$.
