BMI stands for Body Mass Index. It is a calculated estimate of your percent body fat. If you were to divide the body, you would have 2 piles of either fat or muscle with organs and bones. A BMI or percent body fat less than 25 is considered healthy. Since your known body fat is lower than this norm, BMI will not accurately calculate your percent body fat. I would suggest you input your body fat data to the Healthy Body Calculator® to get a more accurate result.
If you want to lower your percent body fat and know that you are 20.6, I would suggest you enter your current body fat as 21 and your body fat goal as 15. The Healthy Body Calculator® will tell you what you should weigh when you reach your goal of 15% body fat.
Your current exercise routine would support a reduction in body fat. However, you did not say what you were eating and food has a great impact on body composition as well as exercise. If you are eating sufficient calories to maintain your weight, preventing weight gain and limiting fat to less than 30% of your total calories, you could reach your goal even without any weight loss. If you also want to lose weight, exercise is the only way to maintain the muscle you have and lose only fat pounds. Unfortunately, I cannot predict how long it will take you to lose 6% of your body fat whether you maintain or lose weight. I can only predict how many calories you need to lose 1 or 2 pounds per week. You can add a weight loss goal along with your body fat data and the Healthy Body Calculator® will predict how many calories you need to reach your goal.