I recommend you stay away from calculating max heart rate with your older /special population clients because they may be taking medications that slow HR. in that instance it wont be accurate and the RPE scale and/or Talk Test is better.
The problem with THR calculations is that they assume the higher the HR, the harder the exercise. while this does make some sense, they don’t take into consideration how the exercise feels. Ive seen people who looked fit and healthy get pooped out by 60% of max HR! In them, even that relatively moderate exercise was perceived by them to be “high intensity.”
the RPE scale / talk test is a much better way to do things because they are more personalized
That said, be aware there is another max HR equation that is a little different than “220-age” that you may want to take a look at. Here is my review of it: