The emergence of steerable flexible instruments have widened the uptake of minimally invasive surgical techniques. In sinus surgery, such flexible instruments could increase the access to difficult-to-reach anatomical areas. However, design-oriented metrics, essential for the development of steerable flexible instruments for maxillary sinus surgery, are lacking. This paper proposes a method to process measurements and provide the instrument designer with essential information to develop adapted flexible instruments for limited access surgery. The method was applied to maxillary sinus surgery and showed that an instrument with a diameter smaller than 2.4 mm could be used on more than 72.5 % of the subject set. Provided that this flexible instrument can bend up to 164.4 • , all areas within the maxillary sinus could be reached through a regular antrostomy without resorting to extra incision or tissue removal in 94.9 % of the population set.