Stock sold but cost/gain not reported on 1099-B. I averaged high/low and came up with cost. Do I enter as the cost basis or as an adjustment on schedule C- what adj code?

[jdfkin]

Sorry meant schedule D. Was a Primecap fund purchased over many years. Few records. Once I determine cost do I put it in column E (Cost or other basis) or Column G on Schedule D?

[DavidD66]

You would report that amount as the cost basis in column E.  It is not an adjustment to the cost basis, it is the cost basis.