A forecast is easier to review when every row carries the method that placed it. The design can be explained without publishing client financial data.
Preserve the source
Use stable keys to join historical and current records. Keep a reference to the source beside the derived row so authorised reviewers can trace it.
Name the allocation method
Store the method used to place each row in a period. Make date assumptions and rounding rules explicit.
Make exceptions reviewable
Route missing dates, duplicate records and ambiguous mappings to an owned review queue. Keep the supporting client records within the authorised system.