I don't think so, DaveF, because you forgot about inactive users. Your suggested requirements should be combined with activity points.
Thinking about it how about "rolling merit"
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You need:
Brand new: Same
Newbie: Same
Jr Member: Same
Member: 10 but must get at least 1 every month or you drop to Jr. Member
Full Member: 75 but must get 5 every month on average per rolling 6 months or you drop to Member
Sr. Member: 150 but must but must get 7 every month on average per rolling 6 months or you drop to Full Memeber
Hero Member: 450 but must get 8 every month on average per rolling 6 months or you drop to Full Memeber
Legendary: 750 but must get 10 every month on average per rolling 6 months or you drop to Full Memeber
-Dave
I think it can be improved like this:
Brand new: Same
Newbie: Same
Jr Member: Same
Member: 10 but must get at least 1 merit for each 100 activity points to maintain the rank, if not, user will drop to Jr. Member
Full Member: 75 but must get 5 merits for each 200 activity points to maintain the rank, if not, user will drop to Member
Sr. Member: 150 but must but must get 10 for each 400 activity points to maintain the rank, if not, user will drop to Full Member
Hero Member: 450 but must get 50 for each 800 activity points to maintain the rank, if not, user will drop to Senior Member
Legendary: 750 but must get 100 for each 1000-1200 activity points to maintain the rank, if not, user will drop to Hero Member