Hi JB,
I do not know what SCM calendar is and how and where does it say week belongs to month m1 or m2
If you have multiple periodicity used for storage buckets, then the 1st week BUCKET OF the month can have variable number of days from 1 to 7 days and the month itself can have variable number of week buckets from 4 to 6. In such a situation iterations may not work correctly all the time, if you are copying the entire month's value to the 1st week bucket, the only way to do it is in a storage bucket view wherein you can put the sum total of all the buckets in the calendar month to the first bucket of the calendar month.
If this is the case then, you need to define layout variables for what month and what # week, and then in next step so the arithmetic based on the value of the variables. I think this will be a macro of moderate complexity.
I am no expert on macros or SAP in general but hope this gives some clues
Thanks
BS