Were these two efforts done on the same course, under similar conditions? It’s important to compare like for like, so you eliminate the variables.
Joe Friel talks about this as a marker for when to move from base to build/speciality phases. He mentions being able to ride at your Aerobic Threshold HR (+/- 2bpm) for the duration of your goal event, before moving up to the next level of training. So if your event is 3 hours, then you should be able to ride at aerobic threshold HR (+/- 2%) for the entire duration. This requires a flattish course, or perhaps in indoor ride.
I had seen the first article alright before I posted. It’s best used so as a comparison tool rather than targeting a specific EF number.
On a slightly different topic. What’s your own opinion on the 5% decoupling ‘target’. In my experience it’s a fair whack of a drift and only something I ever see if I’m coming back from illness or a long break or both.
Personally (and my coach says to me), metrics like EF, Decoupling, etc which takes account HR values, are very hard to track and compare. Simply because there are too many external factors which have influence on HR. Like temperature, dehydration, fueling, sleep, mood and many more.
You can track mean values over a few weeks time, and maybe it can show you some trend. But I think comparing these values between two activities won’t give you any reliable results.
As Someone_Else has said, HR is affected by many outside factors. I do compare activities that are “prescribed” under the same conditions and route (if outside), but also look at RPE/Feel and Power along with HR. If weather was different, there might be the same power:Hr but speed is different. Lots to consider, and not always comparable.