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Here's how to do the BMR calculation manually.
Take your lean weight and multiply by 1.15 for men, or 1.20 for women. That gives you a "typical" overall body weight in pounds. Calculate your height in inches. Now, your "Base Metabolic Rate" (what you need to burn just lying in bed all day) can be estimated with the Harris-Benedict formula:
Men: BMR = 66 + (6.23 x "typical" pounds) + (12.7 x inches) - (6.8 x age)
Women: BMR = 655 + (4.35 x "typical" pounds) + (4.7 x inches) - (4.7 x age)
The 66 and 655 aren't typos. The BMR for women is less dependent on height and weight, which is why BMR is never less than about 1100 calories a day. At the risk of being pedantic, if your calculation looks wacky, make sure you're doing the separate multiplications in the parentheses before you do the final tally, make sure to subtract, not add, the "age" term.
The "rule of thumb" calculations are based on 9-11 calories per lean pound of bodyweight for fat loss subject to certain minimums. Because women in particular may be under 100 lbs in lean weight, the calculator imposes a global minimum of 1000 calories. For muscle gain without fat loss, the rule of thumb is based on 15-17 calories per lean pound with imposed boundaries of 1.2 and 1.6 times BMR. These ranges are based on the available research literature, as well as detailed comments I've received from hundreds of you. Again, there is no "exact" figure, both because individuals vary widely, and because the body is very efficient at changing its activity level in response to minor variations in caloric intake. So the portion rule will serve you well, so long as you're not significantly outside of the ballpark intakes here.