Weight loss is all about losing fat and preserving muscle
BMI is inherently inaccurate when considering muscle mass because it ignores muscle mass. A lean 5'7" bodybuilder weighing 240 lb at 5% body fat would have the same BMI as as someone with no muscle weighing 240 lb at 50% body fat. For greatest accuracy you should use body fat percent as your goal and choose a target of 15-20% if you are female. Learn to use calipers. It's cheap, practical and accurate.