A heart rate monitor will probably provide a better calorie burn calculator providing 1) that it is half decent one and 2) you set it up correctly 3) you remember that you do actually use calories up just existing and it will count them as well remember to deduct your existence calories when calculating exercise calorie burn.
Calories are not the be all and end all you could probably eat only mars bars and remain within your calorie count but your diet would be shocking and the weight loss temporary. The important bit is clean well balanced nutritional food and this where the apps really come into their own They will highlight the worst areas of your diet and where you need to tweak them. You can also never out exercise a bad diet yes it may work for a time but as I found out you will reach a plataeu and no amount of crunches or exercises will shift that last bit. Also as you lose weight and gain more lean muscle mass you may have to increase what you eat to lose more weight as your body becomes more efficient at converting clean carbs into energy. It sounds counter intuitive and is far too complicated for me to explain coherently without typing straight out of a book but any decent sports nutritionist or proper nutrition book would be able to give you good guidance in this regard. Just look at what the TDF riders eat and how skinny they remain even off season.
Re getting a decent HRM I bought a
Decathlon one for £35 and on a couple of occassions it showed my max heart rate as 282. I did not recall chasing Contador up Mt Ventoux at the time.
EDIT: this post is related to my experience at 40. If I am not careful I do tend towards chubbiness when I turned 35 I really had to begin to watch what I ate. When I was 18 I was 10.5 - 11 stone and could eat and drink whatever I liked or so i thought maybe my body is now paying me back. Everyone is different though