Calibration problem with 4 wide-angle cameras

[FookSong]

HEY man，
I think you really did a good job in multi-camera calibration!
While,I use your toolbox to do the calibration for my 4 wide-angle（160FOV） camera system（each faces one direction）on the box and the size of my box is 252010cm.
I did many many times and always got box of 252510cm, which is different from the original box size. （I use sqrt( PATTERN_AREA(cm^2)/PATTERN_PIXEL_AREA(pixel^2) ) to achive scale）.
Can you give me any advice to improve my experiment?It really stucks me a long time.

Thanks!!!!!!!!!!!!!