You should always use the double date format to avoid any confusion between before or after midnight. You should be consistent, whichever time you use. The precision of your recorded time depends on the type of star. See Table 6.1, in Chapter 6 for guidelines. When in doubt, it never hurts to be more precise.
Many observers record all their observations to the minute regardless of the variable type.
The S stands for Standard. The other kind of chart issued is a Preliminary, and this would be noted on the chart itself. New observers should avoid using the preliminary charts. The B stands for the b scale of the chart. Had Z UMa turned out to be very bright, you might have used the a scale chart.