HRM and MFP calculations and Machine Calculations

Hi everyone! I have used a heart rate monitor for a few years. My old one (which I lost) was one of the polar one that also kept track of calories which I loved. I lost it, bought a new one a year ago, and realized it didn't have calories which didn't really bother me until recently. I've mainly been working out for a certain number of minutes, doing intervals, and really just keeping my target heart rate in the ranges that I've been aiming for. However, I am NOW wanting to be diligent about tracking pretty accurately (as much as possible) the amount of calories I am burning. Up until this point I've mainly been logging my food choices into MFP. So my question is, should I type my weight into the cardio machines and go off of that...even though I've heard those aren't the most accurate? Also how accurate is MFP for the calculations on things that don't have a machine to gauge for example when I take a spin class or something that doesn't allow me to input my weight etc. OR should I just suck it up and spend the hundred bucks and buy a new HRM like my old one, and keep this other one as a back up? Your thoughts or experiences?? Any other ideas?

Aspen