If that Full member has not sent out any Smerit up to that point they would have 74 75 Smerit
What you do mean by that? There were some default merits and sMerits, which were distributed when Merit System were introduced, so those 250 are probably default merits (does not mean he should have 74 smerits).
sMerits increased only when you receive merits from another member.
This was using OP's example of a Full member acheiving Senior Member status. Full members regardless of activity started with 100 Merit score.
They then needed to be given 150 Merit to reach senior member, of which half would be 75. I did not notice my typo in my post will edit.
Not knowing their previous activity prior to Merit I can't say if they would have been given any Smerit on the day the system was introduced.