I'm thinking the circumference of your chainring is (#teeth/2) inches, so the change in circumference would be 1/2 inch. As for diameter as a function of teeth, circumference equals 2*pi*R, so the radius R = #teeth/(4*pi), and thus diameter D = 2*R = #teeth/(2*pi). If you add one tooth, the change in diameter would be 1/(2*pi).