If you can afford it you should spend roughly around $100 and get a Heart Rate Monitor. I think the ones with a chest strap are most accurate. The machines can either overestimate or even underestimate. I notice that wheen I compare my HRM to the machines and depending on how hard I've worked it's gone either way. I have a Polar F6 and absolutely love it. You input your gender, age, height, weight, resting hear rate. Mine was the best investment I've made yet. Hope this is helpful.
Ann Marie