I put it just under my pecs...you may want to try that. No matter how big the man boobies (be it muscle or fat)...most of use have a flat spot just underneath them.
As for the calories burned...I don't know. I've seen some goofy stuff with May My Ride and Strava...even with a HR monitor.
I look at it this way, if my target HR is 150-180 and know (with proof from a HR monitor) that I'm staying in the zone giver or take the duration of my ride...I'm cool with that and I'm pretty darn sure I'm burning the calories...