Pat,
The first arrows I used with my Bear Black Bear recurve in the 1970's were Microflights.
According to the reference section in "Shooting the Stickbow", #9's are rated for 55-60 at 28"arrow length with a 5lb range difference for each inch change in arrow length: 29" 50-55, 27" 60-65.
Back when these were popular 125gr was the most common point weight and Dacron was the string type so these weight ranges would most likely be based on those parameters. Changes to string type, point weight, bow efficiency, and form would affect tuning these as with any shaft material.