  We empirically assess estimates from v3.0 of the JWST NIRCam Exposure Time
Calculator (ETC) using observations of resolved stars in Local Group targets
taken as part of the Resolved Stellar Populations Early Release Science (ERS)
Program. For bright stars, we find that: (i) purely Poissonian estimates of the
signal-to-noise ratio (SNR) are in good agreement between the ETC and
observations, but non-ideal effects (e.g., flat field uncertainties) are the
current limiting factor in the photometric precision that can be achieved; (ii)
source position offsets, relative to the detector pixels, have a large impact
on the ETC saturation predictions and introducing sub-pixel dithers in the
observation design can improve the saturation limits by up to ~1 mag. For faint
stars, for which the sky dominates the error budget, we find that the choice in
ETC extraction strategy (e.g., aperture size relative to point spread function
size) can affect the exposure time estimates by up to a factor of 5. We provide
guidelines for configuring the ETC aperture photometry to produce SNR
predictions in line with the ERS data. Finally, we quantify the effects of
crowding on the SNRs over a large dynamic range in stellar density and provide
guidelines for approximating the effects of crowding on SNRs predicted by the
ETC.
