The OP doesn't know how much the sources have spent from their allocation vs their earned sMerits, so I'm guessing all of their spending will be subtracted making merit sources less likely to appear on this list.
Op can't know how sMerit have sources or how much airdropped merit a source had in the past since those data are private and can't be find anywhere (They can't find only log in the account, maybe admins can see these data with admin privileges).
The only thing that Op can do is:
[Received merits(-1 if odd number)/2] - Sent merits = merits on Bags.
Yes he needs to put -1, since every 2 merits we have 1 merit generated and we can't have 0.5 merit on bags.