Are there numerical values for the monitoring zone cutoffs on ASQ-3? On one of the age intervals, the gray line indicating the monitoring zone ends halfway in the circle representing 45; does that indicate that the monitoring zone cutoff is 42.5?

The monitoring zone for ASQ-3 is set between 1 and 2 standard deviations below the mean for each domain. Table 18 in the technical appendix (available online or on page 171 of the ASQ-3 User's Guide) includes standard deviations for each domain of each interval. The numbers in Column 1.0 SD represent the top end of the monitoring zone while the numbers in Column 2.0 SD represent the cutoff score, which is also the lower end of the monitoring zone. 
 

On the Information Summary sheet, the middle of the bubble represents the score of the bubble. So, in your example, a gray line that runs down the middle of the 45 bubble means that 45 is the top end of the monitoring zone. If the monitoring zone started at 42.5, the line would appear midway between the 40 and 45 bubbles.