I am lucky because I am broke. So any flights I take with miles are flights I never would have been able to get regardless of what the cpm is.
But I still look for 2 cpm at minimum on my awards and since I travel so infrequently and mostly international I am usually around that number.
It is also the intrinsic value of the flight. Intangible factors that make this trip PERSONALLY more valuable because many times cpm does not reflect the impact this trip has on your quality of life.
It is a general rule of thumb but not something to beat yourself up about if you are a 1/2 penny less.