Have a look on the web for max hr test to see ideas for the stress test to get your heart to its max level, and then use this as a basis for your calculations. The generic 220 - age formula is a guide, and originally was caveated with a +10/-10 % rule ( may even have been more than 10%). Then you take your hr reading divide it by your max hr and then times this by 100 to get your hr %.