Sorry I missed a sentence. I found the formula by printing a grid of holes that had different sizes on one axis and different $fn on the other axis. $fn can always be lower than the formula but when higher the holes start to shrink because the vertices are too close for the tightness of the corner. If I want the hole to look round and touch a cylinder in many places I set the twist parameter to 2. The alternates the vertices but also offsets them a bit more so they spirale around the hole. Alternating them gives a good connection to the infill and the spiral makes it look round.