Last years' sales belong in a different table, say SalesHistory - with a column for time period, in your current case - year. This makes both tables narrower and allows you to easily compare any forecast to any previous years' actuals. It also facilitates changing the granularity. You can see where this is going - sooner or later the business will want forecasts by month or even by week.
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Understanding and using APPLY, (I) and (II) Paul White
Hidden RBAR: Triangular Joins / The "Numbers" or "Tally" Table: What it is and how it replaces a loop Jeff Moden