I don't do actual mileage runs, but when I'm taking a trip primarily for the miles, I look for $0.04/EQM or less. If I am planning on a trip or otherwise find the opportunity interesting, I'll accept a higher cost.
My just-completed run to SYD on DL came to less than $0.02/MQM with the double MQM and my weekend trips to Europe last year ranged from $0.026-0.041/MQM.