Split your run in half, by time or distance.
If you split your run by time:
For each half divide avg HR into distance (distance/avg HR).
Then subtract the first half quotient from the second half quotient.
Then divide the difference by the first half quotient and you will get a percentage.
If you split it by distance you can use time/avg HR. The difference is your answer will likely be a negative number, unless your aerobic fitness is that extremely high.
A couple flaws with this.
1. Heat and high humidity can cause the percentage to be high, due to cardiac creep.
2. Try to run on a flat course or run 2 loops around the same course. If one half has more uphill then the other this can screw up results.
3. The warmup can cause the results to be higher. In WKO+, on a 2 hr+ run, if I counted the first 15min I got over 8% decoupling. By not includeing the first 15min, it was only a little over 6%. I am trying to find out if the 5% allowance is to account for warmup, some creep or both.
Future running partner.