One of the co-requisites for the Rank is Activity, which is not the same as the number of posts. Trimming all the little details relating to how Activity gets built-up, in general terms, you can get a Maximum of 14 Activity points every 2 weeks (see the details in this post).

In the example you propose, the 20 weekly posts would translate to 14 Activity posts every 2 weeks, so you’d really only have 350 Activity points after those weeks (50 weeks -> 25 Activity Periods; 25 periods * 14 Activity/period = 350 Activity points).

An overall merit per post ratio of 1 for a given account is not that easy: mine is 0,7515, so still far from that mark. Regardless, the Activity explanation above likely changes the perspective on the Merit/post ratio, since posts are not the same as Activity.