I think you have to go more by how you feel and how comfortable you are in your clothes and skin. I weight 155 with a BMI of 25.6. According to that I am still overweight however I feel great, look great an have several doctors that I work with telling me i need to stop loosing weight. I also am 5 feet 6 inches so I think if I had a BMI of 20 I would look like a bone rack and probably not feel that great. Remember that muscle weighs more that fat. I have alot of muscle on me right now so I am not concerned with the BMI and don't think that it is an accurate estimate of what is healthy for us. It is a tool but their are so many other things involved. Go more by how you feel , look and how your clothes fit.
Good luck
Debbie