I recently watched Robin Glover's excellent talk on deep sky imaging with one shot color cameras. I also read his SharpCap forum write up on setting the exposure time for a site's sky brightness. Robin also gets into the mathematical justification in the write-up which I like. It seems that for my typical F/4 configuration with the Starizona Night Owl focal reducer, exceeding 30sec sub frame exposures offers no signal to noise benefit. This is a real eye opener. Instead, best practice is to collect as many sub frames to achieve a certain total exposure time. For example 120x30sec exposures for an hour. These concepts were played with the six images below from the most recent two imaging sessions. 

NGC 281 (Pacman Nebula)

NGC 6992 (Eastern Veil Nebula)

NGC 7635 (Bubble Nebula)

Messier 13

NGC 7023 (Iris Nebula)

NGC 7380 (Wizard Nebula)