  We investigate the relation between rotation periods $P_\mathrm{rot}$ and
photometric modulation amplitudes $R_\mathrm{per}$ for $\approx 4,000$ Sun-like
main-sequence stars observed by Kepler, using $P_\mathrm{rot}$ and
$R_\mathrm{per}$ from McQuillan et al. (2014), effective temperature
$T_\mathrm{eff}$ from LAMOST DR6, and parallax data from Gaia EDR3. As has been
suggested in previous works, we find that $P_\mathrm{rot}$ scaled by the
convective turnover time $\tau_\mathrm{c}$, or the Rossby number
$\mathrm{Ro}=P_\mathrm{rot}/\tau_\mathrm{c}$, serves as a good predictor of
$R_\mathrm{per}$: $R_\mathrm{per}$ plateaus around $1\%$ in relative flux for
$0.2 \lesssim \mathrm{Ro}/\mathrm{Ro}_\odot \lesssim 0.4$, and decays steeply
with increasing $\mathrm{Ro}$ for $0.4 \lesssim \mathrm{Ro}/\mathrm{Ro}_\odot
\lesssim 0.8$, where $\mathrm{Ro}_\odot$ denotes $\mathrm{Ro}$ of the Sun. In
the latter regime we find $\mathrm{d}\ln
R_\mathrm{per}/\mathrm{d}\ln\mathrm{Ro} \sim -4.5$ to $-2.5$, although the
value is sensitive to detection bias against weak modulation and may depend on
other parameters including $T_\mathrm{eff}$ and surface metallicity. The
existing X-ray and Ca II H&K flux data also show transitions at
$\mathrm{Ro}/\mathrm{Ro}_\odot\sim 0.4$, suggesting that all these transitions
share the same physical origin. We also find that the rapid decrease of
$R_\mathrm{per}$ with increasing $\mathrm{Ro}$ causes rotational modulation of
fainter Kepler stars with $\mathrm{Ro}/\mathrm{Ro}_\odot \gtrsim 0.6$ to be
buried under the photometric noise. This effect sets the longest
$P_\mathrm{rot}$ detected in the McQuillan et al. (2014) sample as a function
of $T_\mathrm{eff}$, and obscures the signature of stalled spin down that has
been proposed to set in around $\mathrm{Ro}/\mathrm{Ro}_\odot \sim 1$.
