There is a quicker, easier and more accurate way to figure out the length needed for a side-branch tube to counteract exhaust resonance.
You need to take a Real Time Analyzer with you in the car. You can get apps for the iPhone & iPad. Using the RTA drive your car until you hit the resonant frequency then look at the RTA to determine the peak frequency. Now all you need to do is use a closed tube calculator to determine the length of pipe needed to create that resonant frequency. Using the same diameter tube as the exhaust cut a hole in the existing system before the resonator/muffler then weld your closed end side-branch tube to the system. Bingo - resonance gone
There are many closed tube calculators available on the internet.