accuracy of heart trate monitors for calories burned

Vealchop1

I picked up a polar ft4 and like it a lot. However I was wondering if anyone knows how accurate the calories burned is. When setting up originally I put in my weight about 10 lbs less than I actually am. I did the p90x core synergistic routine and it said I burned 572 calories ( I usually run in place when they are between exercises). For this site, I usually only put down 70% of calories burned just to be safe.