would you mind to put an example data here?
i think, if you want to know the growth of your product then just subtract last year product with current year product.
e.g.:
thediff = Product_PY - Product_CY
but if you want to know the average of the growth between past year and currentyear:
theavg = (Product_CY + Product _PY) / 2
and if you want to know the percentage for currentyear:
theCYprctg = (Product_PY - Product_CY) / Product_CY
and percentage for past year:
thePYprctg = (Product_PY - Product_CY) / Product_PY
hope it help.