Your snarky remark is completely incorrect. ABT investor relations will publish their official info soon on their website. Its tentatively looking like 52.29% ABBV and 47.71% to ABT, but that is just how Fidelity allocated it. I would wait for ABT's guidance if you want to do it correctly.
As of two minutes ago, here is what is posted on ABT website: "For shareholders requesting information on cost/tax basis information: Cost/Tax basis information for Abbott and AbbVie shares will be available on our website shortly. We appreciate your patience."
That statement was up on the ABT website last night too. Shortly means different things to different people. All of the price data is in. I think they can choose one of a few methods, though there may be a commonly-chosen method now that the brokers report basis information for newer acquisitions. I wonder what they are waiting on. It could have easily been computed an posted Wednesday evening after market close, since no normal method uses data past the close on the first day of exchange trading.
On the other hand, the fact that they will post this is to their credit. Many companies do not care enough to help their shareholders, preferring to suggest that people consult their tax advisers.