Hi Chris,
Glad to help.
For your 1st Q, I took around pass 21:00 hrs local time. As final daylight fall off the horizon pass 20:00 hrs, it would be better to take pass 21:00 hrs to show the full blow of the stars. The later the night, the greater the intensity of the stars up at the sky.
As for your 2nd Q, trust me, it is just plan pot luck as I only wanted to try out the star trail as it was my first attempt.
As what "giantcanopy" and "weixing" have mentioned, you can read up more about it. Maybe a good start would be Wiki. I've actually learn quite a lot from the lead provide by "giantcanopy".
Wish you a happy and smooth holiday as the summer season have started and the weather should be much better than the period I've visited.
