Originally Posted by
Rob_E
Figure out what temps you're comfortable at. Figure out what temps you'll be sleeping at. Plan accordingly.
+1
I will also suggest that Weatherspark.com is a great way to figure out what kind of conditions are possible and where.
I always try to have a bag comfortable at or below the expected mean low temperature and make sure I can get down to record lows when it is supplemented with liner and clothing. Keep in mind that you have to account for the highest elevations that you might camp at.