In order to earn the "Ideal or Improved BMI" measurement reward within a month, you must record validated weight measurement that is within the ideal BMI range, or categorically improved BMI measurement from a previous value.
The ideal range for BMI is 18.5 to 24.9. For more information on the other BMI categories, check out the "What are the category values for BMI?" article.
Check out your "How to Earn" or the "My Rewards" page on Virgin Pulse member website to see if the "Ideal or Improved BMI" reward is offered by your sponsor, and see other rewards available for measurements!