Philip (and others), the 'most important' takeaway is to find out your, actual, 'max' heartrate.
This is mentioned right at the beginning of the article. You state your 'theoretical' rate, which is exactly that, theoretical, based on a 'guideline' that is well known, but only a guideline.
There may well be scientific research on _actual_ max HR's. I was on a Neilson holiday a few years back, and the gym instructor was doing her science Masters partly on this, and was asking guests to do an exhaustion test on a treadmill, to establish Max HR. (a controlled, and gradual, increase in speed and inclination) I don't have other examples, but mine was 199 for a 39 yr old male.... so a substantial amount higher than 'guide'.
I typically find I run at 175, when doing 8min30 pace.
Once you have this as a starting point, you can use the percentages with confidence, although I suspect that even there, your 'most suitable' percentage may vary.