24 x 36 tank is ~60 gallons. A standard sized FB would need a throat of ~40 sq in. I'm guessing you're using a cylinder, not a tank with belled ends. In that case your numbers make sense. 2.59" would be the correct BP gap.For example, the tank that I'm working with is 24 by 36, my Throat area is giving me an opening of 46.66, 3.85 inches (went with 4). So if I'm wanting to match the area so that the flow of heat is steady around the baffle plate I need to match it.