Confused

If you set MFP (or whatever calculator you are using) at sedimentary with no exercise and set it to 1lb a week loss (or whatever you choose) you'll get your starting calories. If that's 1200 then you exercise and allow MFP to add those calories back in you can certainly eat some of those calories back. Just don't eat them all because honestly the fitness trackers, even ones with the HR monitors like Polar aren't all that accurate. If you're using a chest strap then maybe you can consider it 90% accurate but that's about the best you can do. So if you do 700 calories a day of exercise, MFP would show you 1200 + 700 = 1900. Allow yourself to eat back up to say 1700 or so and try to leave yourself still under your final goal and you'll be fine. Watch the weight weekly for a while, if weight loss stops or goes the wrong way then it's a sign that your fitness tracker is severely off and you need to not eat back so many of those calories.

I use a Polar H7 strap. It's pretty much dead-on accurate for heart rate, but all apps are different. I find, for myself anyway, Endomondo to be the most accurate for me, the Polar app itself seems to over-estimate calories.

The mistake some people make is that they use the wizard on MFP, or a weight loss calculator like iifym.com has and they include their exercise in the calculations. Then they allow MFP to adjust their goals as they exercise so when they eat back their exercise calories they are over-eating. That's why I always figure my calories as sedimentary and with no exercise first to give me a base number to start with. I have a desk job, so if you have a manual labor type job (factory work, on your feet all day, etc.) then adjust that accordingly.

Hope that helps.