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Lowest merit to post ratio, being that of cero, is what the vast majority of Bitcointalk accounts have. Bear in mind that, currently, there are around 29.909 accounts with at least 1 earned merit (I assume that is the context – excluding airdropped merits), out of a total of 2.651.150 accounts … Of course if you delimit the scope to a month (having made at least one post in that timeframe) the user base is significantly smaller.
The net (created-deleted) number of posts in a month can be retrieved as a comparison between 2 profile data snapshots one month apart, or going through (scraping) their historical posting data, but as I said, the lowest merit/post ratio is probably not a great one to chose, as most accounts are there (with a cero value ratio).