Is there a way to view the opt-in rate via the API? I have been unsuccessful in finding one.

I am trying to retrieve the numbers displayed in "App Analytics" under "Metrics" > "Active Devices" using the API. Since there does not seem to be a dedicated report for this metric I am using the daily "App Sessions Standard" report and by looking at the "Sessions" field I am able to recreate the numbers displayed for the "Sessions" metric. However if I try to infer the active devices by looking at the "Unique Devices" field my numbers for individual days are in the same ballpark (deviations may be attributable to privacy reasons?), but the total number of devices is way off. Is there a better way to recreate this metric?