[Aavso-photometry] Negative CCD observations

[Robert J. Modic]

I was wondering what the best method is for making negative 
CCD observations.  The eyeball method would be the easiest, 
but somewhat subjective.  I'm looking for a quantitative 
method to find the limiting magnitude for a particular frame.
I read that the rule-of-thumb is that a SNR of 3 is roughly 
the threshold brightness for a star.  Can I just measure a 
comp star with a SNR of ~20 to 50, divide the comp star SNR 
into 3, convert this result into a differential magnitude 
and then add it to the comp star magnitude?  

Something like this:

STD+(-2.5*Log(SNR3/SNR20))

I could then average the results for a few comp stars to get 
a better result.

Does this make sense or is there a better way to do this?

Bob Modic