I had issues with the belt tensioning app not detecting me strumming the belts consistently. Sometimes it works, but mostly not. But this is most likely entirely up to my phone (Google Pixel 6).
As a workaround I used https://onlinetonegenerator.com/ to play reference tones and I just tuned the belts using my ears.
The upper one should have 86 Hz, the lower one 82 Hz.
And both belts influence each other, so you will never be able to have them both perfectly in tune, but a few Hz up or down don't matter at all.
Kept getting 104 or 105hZ, belt too tight on Prusa App/Web. I tapped the X cross member instead of the belt and it has a resonance at 104Hz ! So tightened until I thought it was about right, then printed a belt tuner. Way better. Prusa really need to come up with something more reliable.
If your gantry seems bend because it has play on one site, don't worry. Don't go bending metal (as some suppose). By turning the 2 screws you can tighten or loosen the tension. Make sure by loosening (or tightening) that the gantry goes into the correct position (both sites no play) Then use the belt tuner to "tune" the belt BUT TENSION BOTH screws bit by bit while measuring. Tightening the left belt will also tighten the right belt thus increase measured tune. By tensioning them both bit by bit your gantry stays in the correct position. Then you'll have a correct Y-axis test.
As a workaround I used https://onlinetonegenerator.com/ to play reference tones and I just tuned the belts using my ears.
The upper one should have 86 Hz, the lower one 82 Hz.
And both belts influence each other, so you will never be able to have them both perfectly in tune, but a few Hz up or down don't matter at all.
By turning the 2 screws you can tighten or loosen the tension. Make sure by loosening (or tightening) that the gantry goes into the correct position (both sites no play)
Then use the belt tuner to "tune" the belt BUT TENSION BOTH screws bit by bit while measuring. Tightening the left belt will also tighten the right belt thus increase measured tune. By tensioning them both bit by bit your gantry stays in the correct position. Then you'll have a correct Y-axis test.