Firstly, I have not gotten the IMU calibration required message. I've got the typical horizon tilt on my P3A...lastest firmwares. I have to tilt the gimbal to -2 to get a good horizon. I tried my first IMU calibration inside my house this morning...the phantom 3 and all components were at room temp (75*F). Once I hit calibrate IMU, the progress bar stays at 0% for 3 minutes or so and then says IMU calibration is good to go (not needed?). The drone is perfectly level during this process. I then tried to adjust the gimbal 3 times and it stayed crooked.

Hello atomphotodesign, do you use IOS app or Android app to calibrate the IMU? What's the app version?

Android, DJI GO version 2.8.1 (339)