I have been reading Ironfit and I am finally going to start training by heart rate zones. I love the book by the way. I also got a Garmin 910xt for my birthday. When I was setting it up on Garmin Connect I set the zone % based on what Ironfit said each zone should be. The problem is that there seems to be a discrepancy between the two. The Garmin is telling me my zones are higher than what the book says. I.e. Zone 2 (based off of max of 185) should start at 139 (75%) but Garmin tells me 151. Is this because the Garmin is also taking into account my resting heart rate? I haven't done an actual test so yes I'm just going off of generalities here. Thanks.
If you want to train by heart rate effectively you need to do some sort of test. You can do a max heart rate test or the BT recommended Lactate Threshold. I started out with Max heart rate test. I did 10 steep hill sprints. My max was 198. I ran for three months at running at 70% max. Then did the LT test and my LT was 187. I now use the BT LT zones for my running. If you just use some general formula you could be training well below or well above the zone you are targeting. We are all different when it comes to heart rate. Just my opinion.