ie for Eta C, try 20 subs at 5 mins @ ISO 100 for example
Yes
IMHO ISO200 will give you as good a result as anything else..the gain in the camera is kept under control and you can stretch in your imaging program.
The more subs the better the signal to noise; the longer the subs the better the build up of photons....so, more, longer subs.
Light pollution is probably the real limiting factor.
Just my 2c