I was fed up with the gimbal horizon issue so I decided to calibrate the IMU and gimbal on a dead level surface but I couldn't find a dead level surface in my own house! So I sat down then stood up and sat again whilst racking my brain on what would be the best way to do this! SO! I chose the kitchen floor and then I began adding paper under the legs until all four sides were dead level. I used a large tradie spirit level on each side to get the levels right. Once I was happy with that, I first calibrated the IMU and then the gimbal. Then I took her out for a spin and low and behold she was solid as a rock! Please see attached photos.