Hi
Your bmi 20.7 comes under ideal range as underweight is less than 18.5..
So bmi 20.7 falls under normal bracket only..
And anything above the cut off ranges indicates risks for certain diseases such as
Diabetes, Hypertension, CVD's, , Alzheimer's obesity, certain types of cancers etc...
Yes BMI is a proper indicator or powerful tool for finding those risk factors associated with various Non Communicable diseases..
BMI cut off ranges
18.5 underweight
18.5 to 22.9 is the most ideal or Normal window.. as per WHO for especially Asians...
So you can check and maintain accordingly..by seeing above mentioned range..
Next Steps
for any details kindly contact for better understanding..
Thankyou