I had a stock that had a 1 for 10 reverse split with an original cost basis of $6894 for 180 shares. What is my new cost basis for the 18 shares I subsequently owned?

Also this stock was owned for over 40 years and went thru 5 different broker agents. The last broker only cover cost basis for the year it had stewardship. Can I use a spreadsheet record of the cost basis increase by reinvested dividends from tax forms?