We investigate stellar magnetic activity from the theoretical point of view, by using stellar evolution models to calculate theoretical convective turnover times ({tau}c) and Rossby numbers (Ro) for pre-main-sequence and main-sequence stars. The problem is that the canonical place where {tau}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}c over the stellar radius for stars of different masses and ages. As Ro depends on {tau}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_X_/L_bol_ versus Ro) with a two-part power-law function. Our fit parameters are consistent with previous work, showing that stars with Ro{less than or equal to}Ro_sat are distributed around a saturation level in L_X/L_bol and, for stars with Ro>Ro_sat, L_X_/L_bol_ clearly decays with Ro with an exponent of -2.4+/-0.1.