I’ve been using RTIMULib, which has a decent calibration routine built in, and have no need to revisit code for the LSM303D. I imagine that it wouldn’t be difficult to modify RTIMULib to use that sensor, though.
My efforts were described here: State of the art AHRS for $25 and I highly recommend this software.
Of course, the accuracy of any IMU depends on doing the very best you can to calibrate both the magnetometer and the accelerometer, and in the latest posts, I don’t see evidence that you folks have really accomplished that. I would like to see before and after calibration plots, showing nice circles (not ellipses) or spheres perfectly centered on the origin.