Use a very conservative estimate of ad inventory.

Inventory is estimated using traffic to GET_listing. This is a pretty good
estimate based on historical trends (compare subreddit traffic to impressions for
promoted links targeted to the subreddit). It seems safer to start out with a low
estimate. This is especially needed because the CPM beta will only show promoted
links on the subreddit listing, not on multireddits or the front page.

r2/r2/lib/inventory.py

@@ -136,8 +136,9 @@ def get_available_pageviews(srs, start, end, datestr=False):
     ret = {}
     for sr in srs:
         sold_by_date = sold_by_sr_by_date[sr.name]
+        sr_ad_inventory = int(daily_inventory.get(sr.name, 0) * 0.5)
         ret[sr.name] = {
-            datekey(date): max(0, daily_inventory.get(sr.name, 0) - sold)
+            datekey(date): max(0, sr_ad_inventory - sold)
             for date, sold in sold_by_date.iteritems()
         }