Ask HN: Estimating App Downloads by # of Reviews

I'm currently doing some competitive analysis and market sizing. Its easy to get some basic figures for websites via Compete and Alexa, but I haven't found a good way to estimate the number of downloads iPhone/Android apps get.

Some apps tell you about how many downloads they have, but was wondering if there is a better way.

One way to guess would be based on the number of reviews (Assuming an average of.5% of users review an app, multiple # reviews by 200).

I'm sure you could nuance this by using other factors (e.g. maybe crappier apps or ones users have paid for have a higher % of users that review).

So, two questions:

(1) Anyone have any data on the % of users that review an app (the app rating would be a useful data point),

and

(2) Know any better ways to estimate the number of app downloads?