Here’s one way to figure a good goal weight, when you know your bodyfat percentage:
Current weight x bodyfat percentage = pounds of fat
Current weight minus pounds of fat = lean weight
lean weight x ideal bodyfat percentage = ideal pounds of fat
lean weight plus ideal fat weight = ideal weight
So, if you weigh 161 and bodyfat is 21%:
fat weight = 34 pounds
lean weight = 127
lean weight x 18% bodyfat = 23 pounds fat
127 plus 23 = 150 = ideal weight
This doesn’t guarantee that 18% bodyfat is right or possible for you (it’s pretty low for most women), and it also assumes your bodyfat percentage is accurate, which they rarely are (if measured properly, it is supposedly accurate within plus or minus 4%, at best). But at least it’s someplace to start.