How much are we talking about in material inventory that has previously been written off? $100, $1000, $!0,000, $100,000, $1Million?
If the amount is substantial then amended returns may be appropriate, to the extent allowed by the statute of limitations.
If the amount is not material then using the zero basis for any of the material used in the production of a product would be appropriate.
You and your accounting/tax professional should establish a "cost accounting" system AND an inventory "method". These are not only defined in Generally Accepted Accounting Principals, but are also spelled out in the tax code. At certain income levels the tax code, as written by Congress, requires the "full absorption method" of determining Cost of Goods Sold (COGS).
Beginning inventory plus
Purchases in the year plus labor and overhead equals
Goods available for sale less ending inventory equals COGS
Gross receipts less
Gross income (for tax purposes)
These are both accounting and tax formulas. The tax formulas are a matter of statute in the tax code.
Again, there are allowances in the tax code for simpler ways to determine COGS and inventory but the depend upon lower income levels. Have this conversation with your Tax professional.