The great year is known very well with good accuracy, as for all other motions of Earth.
Any chance that you could explain just how accurate this knowledge is? Or, are we just expected to take your word for it?
You can start with the Rate of Precession. Is it constant, speeding up or slowing down?
Then, lead into the number of measured Great Years and the methods used to calculate the verified number.
Please keep to the Scientific Method with your data being measurable, testable and repeatable.
Can you do that?