Well about this I think I can help you, I've achieved a perfect level horizon with these steps:
- take a square flat table and make it level perfectly with paper on the corner or what you prefer. Measure with a level straight and diagonal to be sure that is perfectly level
- put phantom on it without battery and with the level on the top of the propellers screw the phantom should be leveled.
- then put the battery in it, with the weight of the battery (needed to do the imu cal) the phantom will not be leveled and you should use paper on the bottom of the legs to achieve a perfectly level phantom
At the end with the table leveled and phantom leveled with battery, start the PC and immediately the IMU calibration. My result has been really great with phantom stable on yaw and gimbal leveled perfectly.
Thank you very much.