Typically you use a soundboard that's 1/2" smaller than your playing surface because this gives you enough room from the edge of the inside of your pot to the edge of the soundboard and it also gives you a big enough shelf for your surface to sit on. If you used a 3.5" surface with a 3.25" soundboard, that would give you a 1/16" shelf for your surface to rest on, and a 1/16" gap between the edge of your soundboard and the edge of the inside of your pot...I've never tried a combination like this but who knows it might work, but I like a minimum of a 1/8" shelf and I also like a distance of 1/8" from the edge of my soundboard to the inner wall of my pots. Good luck with your calls!
