how do you determine how many calories should burn off at th

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lets say you ate1800 and your MFP is set for you to lose 1 pound a week.

I put my activity level as sedentary. Its says i should only eat 1310 a day. Then I add in my exercise calories.

so should i burn off i burn off 490 calories to equal my 1310 calories a day or should I just burn off how many calories that I want at the gym.