I guess your 10% could be underextrusion. Have a look about 
No. Been 3d printing a while. A long piece 290 mm long is short by 2 mm. Has to be a varience in the drive.
Specification is 400 steps/mm
OK, change the 400 steps per mm to be more precise with M92, 402.8 in the case you stated above.
I’m not sure I 100% understand what your issue is. Are you saying that the pitch of the leadscrew is changing throughout its travel and cannot be corrected with M92? In that case get support to replace your leadscrew, that would be a defect.
Hi, I am running into a bit of issue with XY calibration. I realized that my circler prints are never on dimension that prompted me to take a look into the the XY calibrations.
I realized that my X is 399.40 and Y is 400.15 instead of the 400 in M92.
the issue i am having is that every time I updated the value using M92, M500 ( to save the value), double checked using M503. every things seems fine. However, when I Power off the machine and did a M503. The XY steps always return to 400. Is the firmware not allow user to change the value of XY ?
thanks