RE: The Karvonen Formula
For the Karvonen Formula, you need to know what your true resting heart rate is: to get this, take a full one-minute pulse rate the minute you wake up on three consecutive mornings (before getting out of bed) and average the numbers.
220
minus age
minus resting heart rate
equals Important Number
(Important Number X 70%) PLUS resting heart rate
equals beats per minute of the lower end of your training zone
(divide this number by 10 for a six-second count, OR
divide this number by 6 for a 10-second count)
(Important Number X 85%) PLUS resting heart rate
equals beats per minute of the higher end of your training zone
(see above for figuring out counts for six- and 10-second counts)
There are formulas, I think, that are even more precise than this, but this is more accurate than age-adjusted THR alone. You're right - age does not denote a person's conditioning level, and the more fit you are the lower your RHR will be.
A-jock