Ok now I understand your problem. Well, the problem with the acoustic measurement is, that there are no hard and fast rules how to do it (at least with the fast speeds). That's because every camera is different and therefore every signal is camera specific. You just have to experiment a bit. Look, which parts of the signal change when you compare different speeds, and which parts stay the same.
Maybe it wasn't the best idea of me to measure between those two peaks as it confuses people. But the point is, it doesn't matter that much whether you measure on the left peak, the right peak, or in between. This is the measurement error, that you have to deal with when using this app. But this error is small enough to still get usable results, and thats the point ! It may not be 100% exact, but it is not arbitrary, because the results you end up with always are in a certain range, which is about 1/3 of a f-stop.