"This might be too complicated though, hm, I need to think about this  Grin"
then
modified version
 stake = ( (merit x B) / (number of post) )+ B

 B = to stake based on rank, only merit and number of post is not constant.

this was it can easily be done using spreadsheet/excel.

let say
B =
Jr. member - 2
member - 4
full member - 6
senior - 10
hero/legendary - 12

let say 1 merit and 5 post
Jr member
stake =  (1x2) /5 +2 = 2.4 stakes
hero
stake = (1x12)/5 +12 =14.4 stakes

the advantage of this post farmer will think twice to post again if they gain a merit.