Get your taxes done using TurboTax

It sounds like you'll have to do some research and math.  It’s your responsibility to keep the records of your transactions. Brokers did not have to do any of this in the past years.  

The reinvested dividends would have a cost basis of the amount of the dividend.

If you don't remember what you originally paid for the rest of the stock you may be able to use Historical Stock Quote Prices - MarketWatch to determine the original value.

You're correct that you can use "Various" for multiple transactions that were not done at the same time.  Be sure you retain a copy for your records of how you calculated the cost basis.

There is a procedure for entering Various into you investment transactions.

1. Open (continue) your return if it isn't already open.

1. Income & Expenses

1. Scroll to Stocks, Mutual Funds, Bonds, Other

1. Answer Yes to Did you sell stocks, mutual funds, bonds, or other investments in 2020?
   • If you see Here's the info we have for these investment sales, select Add More Sales.
2. Answer Yes to Did you get a 1099-B or brokerage statement for these sales?
3. At Let's get your tax info, choose how you want to enter your 1099-B (type it in yourself)

1. Select the pencil to work on the transaction
2. In the date area there are two bullets.
   • On one Date, and
   • Something other than a Date
3. Select Something other than a Date
4. There is a drop down menu for Various.