Update: Due to the interest in this model I made some aesthetic updates (to make it more readable) in addition to shaving off 0.15mm of thickness (which puts the gauge nearer to the center when the belts are adjusted properly).
If you are anything like me, you struggled to use the Belt Tension app or stroboscope-based method to get your CoreXY dialed in. I made this gauge to get me really close to ideal (which makes the official processes much more tolerable). I hope it helps others… STEP file included if you want to make changes.
Print at 0.15 layer height, structural profile, 100% infill, max print speed of 100 mm/s, with Prusament PETG filament.
It is recommended printing two gauges and fitting them to both belts (adjusting one belt affects the other).
Your mileage may vary BUT my “perfect” settings ended up being near the center (see picture). Don't forget to also square up the gantry: Adjusting Belt Tension (Core One)
The author marked this model as their own original creation.