Ask HN: Rule of thumb to measure the MAU/DAU for Mobile Apps

As an analyst in a tech research firm, this is a question that that I keep encountering and I must confess that I just haven't been able to answer this convincingly. Ideally I assume that it would be a combination of the below: Current MAU = "Total App Downloads" + "Release Date" + "App Category Customer Churn Rate" + "App Platform (iOS/Android/Windows)" But none of these data are directly available. Would be great if someone could throw some light on how to approach this.