It sounds very “scientific” but it is sloppy and not as scientifically justified as gyms and trainers seem to think it is. The problem is that individual’s maximum heart rates at whatever age are poorly predicted by that formula. And of course there’s big difference is the wisdom between someone who’s 65 and been running 10K’s all her life running at 85% of her maximum heart rate and someone who’s 65 and been a couch potato and obese going out looking at that chart and trying to run sustained at what the chart says is 85% of his maximum heart rate.

Better to forget looking at heart rate charts, and perhaps even ignore taking your pulse rate, and instead to use this rule-of-thumb:
When you reach a sustained level of speed that makes you just breathless enough that you would find it uncomfortable to carry on a sustained conversation with a person moving alongside with you that’s about (a) the maximum safe level for many persons, particularly if presently not highly condtioned, and it is just above the level that will give you optimal heart and lung and circulatory conditioning. Of course lower levels can also be highly beneficial, and higher levels will be beneficial for those in good health training for competitive sports.

Keep it simple: Safe heart rate + calories burned = fat loss.