Usually this would be spelled out on the Shareholder FAQ at the Abbott or Abbvie web site, but it isn't. Just rang IR and left msg suggesting they define cost basis split on Shareholder FAQ. For now, I'm splitting my original Abbott basis evenly between ABT and ABBV.
The split for these spin offs has to do with the trading on the first day. There are a couple of ways of doing it. I thought I saw somewhere that they were going to provide info. The cost basis is just a percentage which then gets applied to each original purchase price. In the oil and gas space COP & MRO both spun off their refining businesses and it was done the same way for both.