Body Mass Index (BMI) has serious limitations and is of limited use when calculating a healthy weight. BMI does not take into account important factors such as gender, bone structure, and body fat percentage.
So when trying to find a healthy weight for myself, I use the Met Life tables, which at least take gender and bone structure into account. As a woman who is 5'7" with medium bones, I should weigh between 133 and 147. My goal is 142 to give me 5 pounds of wiggle room before going over my high. Going by BMI alone I'm allowed to weigh up to 159, which is way too much for a female with a medium frame. 159 is better for a large framed man.
Check out the Met Life tables.