When to recalculate TDEE and BMR?

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I've been losing weight slowly for half a month now (woot!) and I would like to know when everyone decides it is time to recalculate in order to get the right amount for a calorie deficit. Or should I just minus a hundred or fifty calories from the total I am using right now? (1800 calories per day at the moment, lost around six pounds so far which is mostly water-weight).