Rocket booster fin size

Rocket booster fin size

How do you calculate the suggested fin size for a rocket booster. Following the formula ((diameter plus 0.5) x length) / 6 makes no sense. For a booster 0.98" in diameter,4" long, we get about 1.9 sq inches per fin surface, 3 fins, but such boosters have fins larger that the sustainer fins. Apogee Sky Metra used as the example.

On the above formula do we divide by 6 for 3 fins (6 surfaces) or by 8 for 4 fins (8 surfaces)?

Thanks, Herschel