I also had the same approach of using the lactate threshold for a more accurate picture. However, Garmin, for example, requires an accurate max heart rate to determine an accurate lactate threshold measurement. They note that if your max heart rate setting is too high, then the lactate threshold test will be too difficult to complete. This is what I experienced, which I wrote about in "What Sports Watches Don’t Tell Women About Their Max Heart Rate." This is why I went back to trying to figure out what my max heart rate actually is.