No, that's not the way it works. Per information on the company website, 58.11% of the original cost basis stays with DUK. Spectra gets the remainder [you'll have to do some math, you know ] It may seem logical to take 1/3 of the basis when you have 1 share spun off for every 2 you owned, but you have to take the relative market values into consideration. Roughly, they take the market price of DUK times 2 and the market price of SE times 1, total them, and then take the percentage of the total amount you got for Duk to that total to get the 58.11%.
There are different, acceptable methods for using the market price. Some use opening, some Closing, and some take an avererage. The 58.11% that I gave you will stand up for tax purposes.
Now might be a good time to ask what you want to use this for. I gave you what you would need for taxes. Figuring return on investment is another thing entirely. For DUK, I would consider the shares of SE received times the opening market, as a distribution in their return. For SE, I would take the beginning market value for my beginning figure.
If that doesn't significantly muddy it up for you, come on back with any questions.
Rip West