如何提高单目相机测量物体旋转角的精度？

Question

I'm facing a problem that although I can use monocular camera and OPENCV to get a cube's rotation angle (pitch yaw roll), but it's get nearly 2 degrees of error per 10 degrees. I use solvePnP to get rvec and tvec, then I use rodrigues to get rotation Matrix, next I get Euler angle from rotation Matrix. This Euler angle is my rotation angle. Now my problem is how can the Euler angle be more accurate, I'd like every 10 degrees' error less than 1 degree. Is there any one have idea How can I improve this result ?