Personally and just my opinion I don't think you can go by the length to tell the weight. Because I see fish(crappie) all the time that just don't have the so called shoulders(thickness) that others have. I don't have a good set of scales myself and I just show pictures of the length of the crappie by putting a ruler in front them. I think that some postal scales would weigh them pretty close to right better than most of the fish scales that I have owned.