You need to get the belt tension in the right ballpark before you use the belt tuning that is built into the firmware. If the belts are very loose, they will respond with a sub-harmonic and make it look like they are tensioned correctly. Prusa has a video with audio that shows someone strumming the belts. You can just watch and listen to get the belts pretty close to correct.
You must align the gantry perfectly with a feeler gauge before you adjust the belt tension and then run the calibration tests. It cannot be overstated how important it is to have perfect alignment of the gantry; otherwise tensioning with the 6.4.0 firmware wizard and subsequent calibration tests will fail. I used a piece of paper to ensure there was no gap between both belt tensioner brackets and the gantry, but you could use a feeler gauge instead. There should be no gap with a gauge on either side when the gantry is forward and at rest (ensure the pulleys are not interfering with the measurement). Tighten the corresponding tensioning screw in half-turn increments until the gaps are gone.
In my case, I had damaged my belts by routing them incorrectly around the linear assembly which caused gaps to keep reappearing during tensioning and calibration. After replacing my belts, routing them properly, ensuring identical numbers of teeth visible on the Nextruder holder, no gaps at the gantry, and tuning the tension, everything started working.
I am having trouble setting the belt tension on a core 1 kit I got several months ago. Updated the firmware to use built in tuner and am getting top resonance of about 95 and a lower resonance around 105-108. Both these numbers seem to satisfy the criterion in the wiki but it gives a Alignment issue detected error and says to check the gantry alignment, which seems fine. Not sure what to do next.
The belt tensioning steps either needs to be re-written like these steps or have an accompanying video. I already got lost at loosening both tensioning bolts and the whole thing came undone.
In my case, I had damaged my belts by routing them incorrectly around the linear assembly which caused gaps to keep reappearing during tensioning and calibration. After replacing my belts, routing them properly, ensuring identical numbers of teeth visible on the Nextruder holder, no gaps at the gantry, and tuning the tension, everything started working.