In your example the difference between maximum BMI and minimum BMI in each range is the same. If that is always the case, you can define a calculation field that produces the same number for all BMI values in a given range:
Int ( BMI / 5 )
You can sort on that field to group values for a summary report. You can also use that field as a match field to a related table for Anesthetic consumption if you are recording only one such value for each range of BMI values.