the 220 minus your age is a good start for max heart rate, it will change a little with fitness, and your general health ( tired , overtrained etc,, )
Agree that most of your training should be done in the 60-70% area, it teaches your body to be aerobically efficient, allows you lots of exercise without making you too worn out
to start with 70-80% should be done conservatively as intervals, ie 30 secs hard, 2 mins moderate, down the track you can do tempo runs etc, but thats down the track
anything above 80%, unless training for an anerobic event ( sprinting, hill climbing ) is fairly counterproductive as it just wears you out
so if the stairs have you at in the 70-80% mark, call them your interval training