I initially used knowledge of my body frame size and the weights of the women in my family who are built like me to average out a goal weight. I then made some adjustments due to activity levels. I'm exactly what my slender grandmother and mother weighed at my age, but I ended up more in the skinny fat realm due to a metabolism impairment. I think a goal composition as opposed to a goal weight is something you work on and reassess as you go since the number on a scale can end up looking like something you didn't expect, for good or ill.
current weight: 99.0