Swapping fitbit for HRM during certain excercises

I don't think you need to remove the Fitbit. Just as long as you enter the start and end time of your exercise, MFP and Fitbit will work together so that your calories don't double-dip.

Are you saying that your Fitbit is inaccurate because you've actually noticed that the readings are wrong, or because you read somewhere that it's not a good tool for that purpose? If your personal readings were wrong, have you tried wearing the Fitbit further up your wrist, and a little tighter than for all-day wear? I find that mine gives very reasonable readings.

If you have entered your activity level as something other than Active, or Very Active, you will get a large number of calories back just for your daily activity. That is as designed. How long are your workouts, to burn 500-700 calories? Not knowing your weight, I can't make a very good guess, but that sounds like an hour at a fairly high intensity?

How long have you been doing it? What have your results been? If it's not broken, there's no reason to fix it. If you don't know if it's even working, why not give it a month and see what the results are?

Be advised, heart rate is correlated with but not a direct measurement of calories burned. It is only a useful metric for steady-state exercise, not sprints or intervals. Heart rate is also affected by a number of other variables, like dehydration or overhydration, heat, intensity of emotion... Being scared makes your heart beat faster, but it doesn't burn a lot of calories.

Your most reliable indicator is going to end up being the result you get over time. If you maintain records of how many calories you consume, and how many calories you think you've burned, you'll be able to compare those notes and adjust your calorie goal as needed. For running or walking, you can manually enter calories pretty accurately with this calculator: https://www.exrx.net/Calculators/WalkRunMETs but I wouldn't even be able to guess how much your classes burn.

How the Fitbit linking works is that Fitbit sends over the total number of calories it thinks you burned in a day. Mfp then subtracts out whatever calories it thinks you burned in a day (which is whatever goes with your activity setting plus any workouts you’ve manually entered on mfp). The difference is your calorie adjustment.

In theory, workouts you manually enter on mfp are supposed to overwrite the Fitbit info for that time period.

So theoretically, you’re not double counting by doing what you’re doing. The workout you enter on mfp is supposed to overwrite what’s in Fitbit for that same time. And mfp will also have them as part of what you burned so they will be subtracted out of what makes up the adjustment.

I’m not sure if you think the fitbit is reading high or low for the workout, but keep in mind that it’s giving you a total calories burned from the moment you hit start to the moment you hit stop-which includes the calories you would have burned had you been sitting on the couch instead of working out. So Fitbit calories reported for a workout tend to be higher than some other tools because Fitbit is intended to be worn 24/7 and the workout info just designates what happened in that time period (not the amount “extra” you burned by exercising).

10k steps a day is considered pretty active (although for your mfp setting it matters how many of those are from non-exercise). So if you’re set to lightly active, I would assume you’ll see larger adjustments pretty regularly. And whether you raise your activity level setting on mfp to get higher base calories and smaller adjustments - or leave it as and have lower base calories and higher adjustments, you’re going to end up at the same number.