HRM vs. BMR

Someone said some newer Polars don't but I don't know.

I think your math is correct. My Fitbit estimates my BMR at about 1 calorie/min (and it's similar to yours overall), until I get down to healthy BMI then it drops to around .8/minute.

If you do want a new device and most of your exercise is something like running, walking, dancing, etc., the Fitbit is great for telling you your total burn across time, no math needed. You can log resistance and activities like cycling.

The number that MFP gives you is your BMR multiplied by an 'activity factor' (see below) based on how active you said you were in your profile, this will give you a calorie value to aim for without having to input calories burnt for exericse. Personally I find that if you say you are sedentary and log your food and exercise properly, the amount of weight you should be losing according to MFP will be close to what you are actually losing.

Also, you are correct that your BMR is often added onto the amount of calories you actually burnt during the exercise, this is usually the case for gym equipment such as treadmills, it may be worth checking the specs for your HRM to confirm if it does this, but if not subtracting 1 calorie per minute of exercise (roughly your BMR) would give you an accurate amount of calories burnt.

'Harris Benedict Formula
To determine your total daily calorie needs, multiply your BMR by the appropriate activity factor, as follows:

If you are sedentary (little or no exercise) : Calorie-Calculation = BMR x 1.2
If you are lightly active (light exercise/sports 1-3 days/week) : Calorie-Calculation = BMR x 1.375
If you are moderatetely active (moderate exercise/sports 3-5 days/week) : Calorie-Calculation = BMR x 1.55
If you are very active (hard exercise/sports 6-7 days a week) : Calorie-Calculation = BMR x 1.725
If you are extra active (very hard exercise/sports & physical job or 2x training) : Calorie-Calculation = BMR x 1.9'

My BMR (according to Klatch-Macardle formula) is 1300. That's 54 calories per hour. So if my HRM says I burned 600 calories in an hour, I actually burned 546? Do I have that right?

My polar clocked me at about that rate sat using the computer (the 54 not the 600 !) so I would say yes the HRMs clock total calories and if you're trying to book them as extra exercise you should subtract the calories you would have burned anyway in that time. Otherwise the BMR / sedentary bit gets double accounted.