The pictures in the instructions are misleading; I noticed that holding the phone in hand causes a lack of readings or readings above 130 Hz. It's best to lay the phone on a heatbed, then it's easy to adjust the belts.
I had a terrible time using the app and website on my phone. It couldn't decide whether the belt was 60 or over 200hz, then it would settle on 181 and remain stuck there. I could strum any belt I wanted, loosen the belt until it's falling off, and the results were the same. I tried with 2 different phones.
Then, I set it up on my laptop with my wireless headset as the receiver. It immediately worked and took less than two minutes to get both belts at 84-86 Hz.
The app seems to have a problem where it gets stuck on a reading. I will pluck a belt once and get a frequency on the app, adjust the belt tension by a quarter turn or more, then pluck the belt again and get the same reading. Gonna try my guitar tuner app instead.
Then, I set it up on my laptop with my wireless headset as the receiver. It immediately worked and took less than two minutes to get both belts at 84-86 Hz.