That's the problem with set equations, they are a one size fit all solution. which just doesnt' work. I would suggest determing what you Anaerobic Threshold is first. Most training zones these days use AT as a marker vs. the 220 - age predicted max hr (which is probably what your HR monitor did...it then takes a % of that Max HR.) Best way would be to get a metabolic exercise assessment (measures your oxygen and CO2 alogn with HR.) Every person is different in what their ranges should be. Only way to know for sure is measurement. I'd check out your local university or try New Leaf: Metabolism, Fitness & Athletic Training
. They are a metabolic assessment company and their site locator will show you where you can get an assessment done. I did and it completely changed my running programs...for the better.