Do you not want to take into account max heart rate? There's a huge variance here - say, anywhere from 170 - 220. The runner with a 220 max HR is going to be chilling at 145, whereas the runner with a 170 max will be going pretty hard!

I agree with the approach, but surely you'd see better results by tailoring it to yourself with blood lactate testing to identify where your aerobic zone actually is - or using one of a number of more fine-tuned methods to guestimate aerobic zones.