According to this FAQ, $13.50 CPM is "a conservative estimate". I recently received my Q1 ad revenue payment and while the FAQ refers to using the "Ad Server Impression" number in the impression report, there is no number in the report with that precise label (and the quotes appear in the FAQ). I assume the "Total impressions" number is the one to use. Using that number totaled for the quarter and the payment I received, I get a CPM of $10.68. That's significantly different than the "conservative" CPM of $13.50.
Anyone else that's using Roku to manage 100% of their ad inventory and receiving reports want to share their calculated CPM?