When I was in treatment when I was abt 15, they used my height and weight pre-ed and from when I was really small to predict how much I should weight when I'm 20. (I'm 22 now)
I was supposed to weigh 142 by the time I'm 20 - I'm 118 now so a 24lbs difference.
If you go to https://www.cdc.gov/...alculator.html and put your pre-ed weight and height from when you were young, it'll give you a percentile (make sure the age is accurate). I then just set the age to 19y 11 months, and tried putting in different weights until I get the same percentile as before.