Keyway centering

[[Fe...]]

"I need to calculate the centering (o alignment) of these slots relative to the main axis. Can anyone help? I tried creating a 3D line connecting the center with the symmetry point of the upper slot, but I'm not sure if I did it correctly."

[[Ma...]]

I think you could use only that point for rotation.
Line is unnecessary for this task.

I would also use symmetry point or plane symmetry, but then use a point from that plane. Vector of that sym-plane can be wrong and is not working for that clocking.
Same is for bottom plane

[[Fe...]]

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  I’m sorry, but I didn’t quite follow your point. Which values should I look at, and where, to understand if it is offset relative to the axis?"

[[Ma...]]

When you are filling alignment, then you can use plane as first element, then circle as second element, then just symmetry point from groove to be used as rotation lock.
Using line from symmetry point and center of a circle is unnecessary step.

Then:

if you would use symmetry from two planes ( of width of groove 16 ) then this can be misleading because it's direction won't be to exact center of a circle.