Does anyone manually calculate their heart rate?

Long story short, a HRM is not in the cards for me at this present time. I would like to estimate my calories burned a little more accurately than what the standard MFP numbers state. I know some say that for an average P90X video, approximately 600 calories are burned, and that entering "Aerobics" or "Circuit Training" in the system will be similar... but honestly, MFP tells me I burn 777 calories doing an hour of circuit training. I'm quite overweight, I understand I'll burn a lot, but that just seems outrageous to me!


Anyway, to get to the point, if I take my heart beat several times during a workout and calculate my rate that way, is it likely to be more accurate than the pre-calculated numbers generated my MFP? My inclination is to say yes, but I wanted to hear what everyone else has to say on the matter.