It looks like you already have all the information you need.
365 * (200 * 60 * 60 * 24) * $AVERAGE_MESSAGE_SIZE * (1 + $NUMBER_OF_REPLICAS) * 1.5
[days] * [messages per day] * [average message size] * [number of times the data is stored in ES] * [magic number for the disk space requirements in an ES index]