Determining needed exposure time given SNR
Here's a simple start, and I suggest you must include your sensor's noise -- not flat-fielding, but just the accumulated readout noise in a pixel.
You can use the Poisson SNR rule for the incoming light, so optical noise $N_o = \sqrt{S_o}$.  Just add the electronics noise, again as a simple model, $N_e = C_(readout) + k\tau$ where $\tau$ is the integration time and $k$ represents leakage current into the pixel well.   $S_o$ also equals some constant times $\tau$ , so just combine the two noise terms in an RSS manner, i.e. $ N_{tot} = \sqrt{N_o^2 + N_e^2}$ .