To do this properly, though, you need to know the frequency of the pitch you are wanting to tune to, not the letter name. I hope you have a frequncy conter, or some sort of device or chart to allow you to do the conversion (there's a program on my computer that does this for me).
Once you know this, the rest is simple arithmetic:
L = [(s/f)/4] - d/3 (for resonators closed on one end) L = [(s/f)/2] - d/3 (for resonators open on both ends) Where: L = Length of the resonator, in cm. s = Speed of sound, in cm/sec (ca. 34,400 cm/sec) f = the desired frequency, in Hz d = is the diameter of the resonating tube, in cm. Example: Say we desire a resontaor for a key with a frequncy of 240Hz, and the resonator is being cut from some 2 inch PVC tubing. First we convert from inches to centimeters: 2 inches = 5.08 cm. Then we plug the rest of the numbers into our equation: L = Length of the resonator, in cm. s = Speed of sound, in cm/sec (ca. 34,400 cm/sec) f = the desired frequency, in Hz d = is the diameter of the resonating tube, in cm. L = [(s/f)/4] - d/3 (for resonators closed on one end) L = [(34,400/240)/4] - 5.08/3 L = [(143.3)/4] - 1.7 L = 35.8 - 1.7 L = 34.1 So a tube of 2 inch diameter, cut to length of 34.1 cm., capped off at one end, will be the correct resonator for the pitch of 240Hz.
Note: I round everything off to the first decimal place. With my hand tools, I can get to the closest millimeter, and that's been close enough for every project I've done so far.