low reprojection error means your data agrees with itself. if the data is poor, reprojection error can still be low. if r.e. is outrageous, something’s definitely wrong. it being low does not imply that all is well. do not do bad things to lower this value. there is no “must be” requirement on its magnitude. larger r.e. just means you need to calibrate better, not worse (board parallel to image plane is worse).
my advice stands.