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Correct, but there is a large obstacle here. The number of post should be the number of posts from a certain date onwards for all users (Merit System kick-off for example). Otherwise, the ratio works backwards for the people who have been on the forum for a long time, since their current post count will be much higher that someone who started posting on or after the 24/01/2018. Unfortunately knowing the starting post count for all accounts is not feasible, since there is no public snapshot from that date (perhaps a backup that @vod may have could serve).
Alternatively, the average number of posts per day could be derived and used, replacing the total number of posts.
For example (roughly), a person with 1200 posts with a 500 day old account has an average of 2,4 posts per day. Let’s also say that the system has been going on for 9 months (simplifying, 270 days), and that in that period of time he has earned 100 sMerits, then his ratio would be:
100 / (2,4*270) = 0,1543 merits per averaged post.
This avoids needing a snapshot and instead uses a moving daily average (I´d prefer the snapshot though for precision). The only thing is that the ratio would be exact for newer accounts, and using a moving average for older accounts that starts farther back in time (posting habits may have varied).