See IRS Pub 550 Chapter 4 for handle reinvested dividends.
Yes the reinvested dividend amounts, from your records, is the cost basis of the shares sold after the first 500.
the reinvested dividends that were reinvested within 12 months before you sold would have to be separated out and the shares purchased within that 12 month period enumerated.
That's the ST numbers and the remainder is the LT numbers.
