There are situations where using half of a round trip price to determine a one way cpp is also not the right thing to do. For example, one search I've done recently is LAX-AKL-LAX in J, departing March 12 and returning April 6. A round trip prices around $8.5k in J. LAX-AKL one way in J is $5.5k, and AKL-LAX one way in J is $2K. I got two IN awards for 100k miles each, and I would say that I am getting ~5.5 cpp.