With a CoreXY you can also use Klipper to compare belt tension between X and Y. This doesn't tell you if the belts are too tight or too loose. It tells you have the belts are tensioned equally.

Run the following commands in the console

TEST_RESONANCES AXIS=1,1 OUTPUT=raw_data

TEST_RESONANCES AXIS=1,-1 OUTPUT=raw_data

SSH into the pi and run

~/klipper/scripts/graph_accelerometer.py -c /tmp/raw_data_axis*.csv -o /tmp/resonances.png

Based off the generated graph, X had much higher frequencies than Y. I had to tighten the left belt tensioner to to bring the frequencies closer to each other.

As you can see from the 2nd test the frequencies for X and Y are closer together, but could still use a little more adjustment. I think a lot of the frequency variation is due to my cable chain. At lower frequencies it rattles considerably more only when calibrating X.