Forget the 220 minus age guff.
No, don't dismiss it as guff without understanding the point of it.
The point is that in the absence of any other useful indicator (i.e. real world empirical data from a run or something) then 220-age is the best remaining option for estimating HR
max.
No other value than 220 in a formula of the form
x-age gives a better fit in a population wide study.
Of course individuals vary, it should be obvious that there isn't a formula out there that can accurately predict individual values for everyone, but if you've got absolutely nothing else to go on then it is the best thing out there.
There are better fit formulae but they are more complicated. HR
max = 208 - (0.7 x age) has a better fit population wide than 220-age.
However, if you've got something that can record HR whilst running, and have no medical reason not to do vigorous exercise (e.g. you've been warned off it by a doctor, or you have a concern over a possible medical condition), then the best way to find your max running HR is:
* find a flat safe circuit (the outside lanes of a proper running track would be perfect) where there are other people about (don't do it completely alone somewhere)
* start by jogging gently for 5 minutes to warm up
* once warmed up increase your speed by 0.5kph every minute or so (if you can't show current pace on your watch then just bump up the speed a bit each time)
* keep doing this until you really can't push it any more, then stop and recover (don't forget to cool down and stretch properly once recovered)
Whatever max your HR monitor hit during the run is now your current best estimate of HR
max. It's not likely to be the exact value but it'll be close enough.
If you then see a higher value in a future run, and it's not just some glitch from adjusting the strap, some weird heart flutter, or passing a bus (this sometimes sets one of my HRMs to read 240bpm) then use that new value, it's that simple.
(The highest HR value I've got when running [sprinting to the finish of a half marathon] is still 13bpm lower than the highest I've ever seen my HR. I get the much higher values during 5-a-side football games and I just can't replicate these whilst running in a straight line[1]. Also I can never seem to get my HR up to either of these levels whilst cycling, even doing hill repeats. I don't want to use the 13bpm higher HR
max value for my running as it would make all of the corresponding 'zones' horrific to train in. Anyway, I digress.)
If you do have any medical condition that could prevent you from doing this, and you need to have an idea of your HR
max in order to know where to limit yourself, then that's a different story. You should probably discuss this with a doctor. From reading the previous posts I would say this would apply to nicknack's situation - speak to your GP first (telephone appointment first given the current situation).
1. I've discussed this with a cardiologist (after having an ECG and an ultrasound - thanks to the Cardiac Risk in the Young charity - back when I was young) and the cardiologist was unconcerned.