It is "definitive" only in that it is ONE way to calculate the FMV. But it is only one way. The form itslf says as much.
It might be the most common way to calculate. It might be the best way. Certainly, since COP has already done all the math for you is the easiest way. But it is not the only way.
If you choose another method, you should be able (and ready?) to defend your method of calculating the FMV of the two post spin parts. But it is not the only way.
As an example, here is another simple way (but probably only applicable to a very few people.) Say you chose to sell all your COP and PSX shares in the market on May 1. You could easily make the case that what you actually got for each was their FMV. You could then take the combined proceeds and calculate the % each issue contributed to the combined value and use this to asign the cost basis. It would be very easy to demonstrate to the IRS that you sold out everything in the open market on Day 1, that that was the FMV. Of course you would have actually had to sell all your PSX and COP on May 1 to do use this method. (Also since you effectively sold out 100% of your original investment basically immediately upon receipt, the tax implications of splitting the original basis between the two new issues was effectively nil anyway. So it really would not matter tax-wise how your divided up your original cost basis.
1) I and others have repeated the 2:1 issue when dealing in price numerous times over a month ago. (In fact even before teh spin happened.)
2) If you will note, in my post you responded to never did I deal in share price it was all referring to a value basis. As such the 1/2 factor is already included. Though I will say that for all my lots, I have calculated a per share adjusted cost basis and that is much more practicable to use XX years from now.