I have been playing around with the image exposure calculator in SkyTools3 Pro and it has enlightened me. You enter in your gear and your conditions and it tells you based on a whole lot of factors the times to image, what your sub exposures should be and what your SNR will be. So I picked the Helix for tonight and based on my conditions (Bortle 7 at zenith, 78 degF, 90% humidity, elevation 22 meters, seeing average), it suggested 293 exposures at 40 seconds for a whopping SNR of 6 with the luminance filter. I don't have the detailed specs on the Astrodon filters yet, so I am using typical filter values that came with SkyTools.
I then decided to see what it would be at Bortle 2 skies with my other conditions the same. It suggested 4 exposures of 28 minutes with an SNR of 19.
The interesting thing is that if I increased the length of the 40 second sub exposure for my conditions, it did not help my SNR. I have always been trying to maximize sub exposure lengths and had used the histrogram as a guide. I don't know how accurately Sky Tools can simulate my actual conditions as the area in the sky that I would be imaging the Helix in is the darkest part of my sky. So I might have to fudge SkyTools results a bit, but it gives me a ballpark number.
The other thing that has me confused is this that for the color filters it chooses different sub exposures for each filter. I thought the idea was to get equal sub exposures and number of sub exposures with the RGB filters if they were balanced like the E-Gen2 Astrodons. If I use different sub exposure lengths and different number of subs, won't that throw off the colors during processing?