The doctors who created the Karvonen method have admitted that they NEVER intended for the fitness community to take that formula as gospel. They came up with it for some patients who had undergone heart surgery, and they needed to give them a 'safe' range to keep their heartrate.
Also, unless you are taking your resting heart-rate in the middle of the night while sleeping, you probably are not getting an accurate rate for that. And, unless you want to spend a great deal of money and go through crazy tests, you will probably never know what your true "max" heartrate is.
Karvonen is a great "guess-timate" for a beginner. But if you have been working out for a while, you will find that if you stick to those numbers, you will not be working hard enough.
For example...
If I use the Karvonen method:
my maximum rate would be 181
60% = 109
80% = 145
When I spin, my average rate per ride is about 167 - with me pushing high 170/low 180s several times throughout. If I quit or slowed down every time the heart rate monitor beeped because I was out of my "zone", it would be like a leisurly riding in the park - I would get a horrible workout.
I think the heart rate monitors are a great TOOL, but shouldn't be the only thing you base a workout on. You should go by how you feel (RPE), and push yourself harder than maybe the day or week or month before. To see how you are progressing - chart your heartrate doing similar activites each time and make your own "range" -- see where it is during warm up, during moderate exercise, and during intense exercise. Then see how that changes the more fit you get.
Just my $0.02.
