Exposure time for subs

I was wondering if I am using a good sub exposure time for my narrowband images. For faint nebulae, I am always using 240s.

I found the CalculateSkyLimitedExposure script in PixInsight helpful and added the camera data (cooled down to -20° C @ gain 121), a background image from my latest session, and a total exposure time of 12000 s (with most nebula, my total exposure time is longer—however I assume that the value should reflect some kind of 'best' value for a specific object to become visible at good quality).

The results for the Anstey limit correspond to my sub exposure time, so I was happy with the results :-)