I found this on a message board after the spin-off.
Before the spinoff, find out what your total cost basis was for all of your shares of the pre-spinoff COP, and let that value be 'B'. Then, the cost basis for COP and PSX is as follows:
COP Cost Basis = (B * .775277816) or use 77.09%
PSX Cost Basis = (B * .224722184) or use 22.91%
COP Closing Price on 5/1/2012 = 56.51
PSX Closing Price on 5/1/2012 = 32.76