You need to calculate the ratio of each stock to the total based on the first day of trading and then multiply times the original purchase price. If you bought more than one year ago it is still long term.
This is the method indicated in the ifo booklet sent by ABT. It specifically stated "the closing price on the distribution date." Since there was not market on Jan 1, 2013, the distribution date, it would seem that the closing "when issue" price would be proper to use. that would be consistant with the immediately previous poster's formula.
I spoke with Investor Relations this morning and, while it may be that they calculate the cost basis allocation along the lines set forth above, they promise that the precise allocation of basis as between ABT and ABBV shares will be set forth in a document they will post of the ABT site sometime next week.