The above is an illustration of how stacking efficiency increases as you increase the lengths of your subexposures. A quick indication of where you are is to look at your Back-of-Camera Histogram with ISO set at 1600 and see what stacking efficiency you can expect. Discussed at length here:
http://www.cloudynights.com/topic/351257-stacking-efficiency-sub-length-skyfog-histogram/#entry4505740