@Mad Mark
"On March 21, 2019 DowDupont was $167 per share."
I'm not sure what this has to do with anything. March 21, 2019 was the "record date" where if you owned DowDupont on that day then you were eligible to receive the spun- off Dow stock, but that doesn't factor into the basis calculation. The calculation starts with your cost basis of your DowDupont stock and the number of shares of DowDupont you owned on that date, before the reverse split.
Take the number of shares of DowDupont you owned on that date and divide by 3. That will give you the number of shares you were eligible to receive including the fractional share.
Multiply your DowDupont basis by 0.33562. That's your basis in the new stock received, including the fractional share.
Now, divide the basis derived for the new shares by the number of shares you were eligible to receive. That's your basis per share in the new stock.
Multiply the fraction of a share you were eligible to receive by the "per share" basis number and you have the cost basis of the fractional share.
