Something to add to the equation, I was listening to Dr. Doug McGuff who wrote a book body by science here http://www.bulletproofexec.com/podcast-26-body-by-science-with-dr-doug-mcguff-md/. (Being interviewes by Dave Asprey) and he said heart rate is worthless for monitoring anything on its own.  To paraphrase him, the most important measurement is "compression" or "pressure", (I forget the actual term he used). He says to find what your heart is truly doing you need that and you could add that to heart rate, but the body will adjust the compression with any elevated heart rate.  So, elevated heart rate does not tell you how hard the heart is actually working because in long term cardio it will adapt and and lower the "compression", thus not helping with increasing your cardio.  He along with others suggest long term cardio actually will decrease your cardio strength. (Kiefer does not like it either). That is a long way to say, it seems unlikely heart rate will help you determine any type of fat burning zone. 

Well... what is the definition of long term cardio? What is the threshold to reach "long term"? I am referring to maybe 45-1.5hr instead of 20-30 minutes of HIITI'm sure the actual contraction of the heart muscle plays a factor and that it does adjust rate and contraction amount to be more efficient. Unfortunately, the aspects of adding this to the equation would be completely make it impossible to figure out what your body is doing. I'd imagine you would need to be tested at rest and in an elevated state such as running on a treadmill. You would need monitoring and maybe even dye to blood to see force (psi) that the blood is flowing at each point in time.Also I'm curious what cardio strength really is? Oxygen efficiency?

I am referring to studies done in the 80s and since I moved homes at least 5x and changed States since then, I don't have those handy BUT, doing cardio at 65% MHR or more for more than 20 minutes begins a slope of diminishing returns.  After 30 mins not much benefit can be gotten in terms of increased capacity, etc. The only thing possible is calorie burning HOWEVER, what gets burned, fat, lean muscle, etc. depends on a whole host of other factors.  Yes, the body does adapt, always, so as you increase in cardio vascular capacity your HR will decrease doing the same work (so to keep challenging yourself you need to increase the load; weight vests, ankle, wrist weights, faster, steeper, etc.) your resting HR will also come down.  As with all these, there is a limit, determined by diet, genetic capacity, etc.  Doing treadmill at less than 60% MHR is basically just burning calories (mostly fat, but the proportion also varies per individual conditions, and diet), the longer you go without breaks (those of you who love doing it for more than 45 mins or an hour) however, the more the body adapts by trying to decrease your lean muscle mass production and increase fat production. Kind of a general response but the principles are there.