The wheel distance is wheel center to center on a 2 wheel bandsaw, or the length of the freely vibration blade section that you are measuring on a 3 wheel bandsaw.
I built a log into the app that should help me trouble shoot it and improve it based on how it is performing for you. To send me a log do the following:
Email screen shots and a description of the issue, as well as what kind of phone you are on to inkleind2303@gmail.com
You should try to pluck the blade every time the dot passes the center, but if you miss a couple or if your timing isn’t perfect, the app should still work.
The phone microphone should be a couple of inches from the blade ideally.
Pinning down an exact amount of background noise is a bit tricky, but when the furnace is running in my shop, the background noise is close to 70 dB, and the app still works, so it is pretty high.
Displaying and giving a sample of the correct tone is on the list for the future, but I want to work out the measurement issues first.