X,Y,Z calibration Question

Hello I have built the Folger 2020 about a month ago and have been enjoying amazing prints!

However everything printed was all relative to itself so dimensions where never important.

I have a project that requires precise sizes to be fitted onto a model I am making.

I printed a 60mm x 60mm x 60mm cube (20% infill)

The resulting cube was 60.4 x 60 x 57mm

57 on the Z

I looked in the configuration for marlin and Z was set to 4000. I change dit to 3840 and then ended up with 58.6 which is better but not correct.

Rather than playing with this number is there a better way to gain the accuracy or other things I can look at ?

Belts are both snug. Frame is Square.

Thanks!

4000 should be the right number for what you have

I'd check to make sure that all 3 jumpers are installed under the Z stepper driver on the control board.
Make sure that you can move the entire gantry up by hand and it moves smoothly and doesn't bind.
Ensure the Z motor couplings are tight on the motor and the threaded rod, and that they aren't slipping.

Along the same lines, make sure that in your firmware that the Z axis feedrate isn't too fast, it could cause it to bind.