The mean level of chromospheric emission associated with magnetic activity in late-type main-sequence stars has been shown to correlate well with a star's rotation period normalized to the convective turnover time scale. This paper presents an internally self-consistent grid of stellar structure parameters (convective turnover time scales, moments of inertia, etc.) for both the zero-age main sequence and for stars evolved well off the main sequence. The value of alpha (mixing-length parameter) which best fits the observed Ca II H and K emission fluxes and rotation periods is found to depend sensitively on details of the adopted B-V value versus effective temperature and on details of the theory. One star in the Wilson survey, HD 124850, is an active subgiant with a surprisingly short rotation period. The most likely model for HD 124850 is that of an early F star, originally, which resided on the main sequence in an inactive mode. It then evolved into a subgiant while retaining its primeval angular momentum, and the lengthening convective turnover time scale of its deepened envelope has induced fresh dynamo-related activity.