1. Belt tensioning

If you're having problems trying to get the xy belt tension(s) within tolerance, take a look on this page. Good instructions to correct xy skewing and other information (including links to download and print improved replacement and add on parts - including the belt tensioner).
https://arachnoid.com/3D_Printing_Prusa_Core_One/

It may help to move the printer away from other electronics, the tuner app was really glued to a 120Hz signal. Prusa, if you see this please allow a way to stopband certain frequencies in the app.

Be careful if you use the app: the instructions in the app are wrong/outdated!

It tells you to aim for 85hz on both belts and to have the same resonance on both belts, which seems incorrect.

The "Adjusting belt tension (CORE One)" article and the tension website both instead say to aim for 98hz on the upper belt and 92hz on the lower belt.

I finally figured out how to use the belt tension website!
I was mistakenly plucking the belt over and over and getting terrible results.
You need to pluck the belt a single time and read the results then pluck again and read the results.
Once I did that it worked like a charm.
Also the gantry is squared with belt tension not force.

!!WARNING!! If your gantry brackets aren't square - which is the case for pretty much everyone seeing as they're made from a metal softer than chocolate - then DO NOT try to straighten the gantry by force without first slackening the tensioners!
IF YOU DONT REMOVE TENSION ALL YOUR PUSHING IS ACTING ON THE PULLEY BRACKETS AND THEY WILL SNAP