Sorry, I'll get back on topic - I've owned a second-hand P3P since early 2017, and recently bought an MPP from the classifieds here. Both were shipped from places far away from my location, and I've never calibrated IMU or compass on either... period. They fly and hover just fine, and before takeoff I always check the 'little red triangle' to make sure it's pointed in the same direction as the nose of the AC... and if I get a compass error warning, I just move the AC.
edited: Actually, I've started using a plastic 5 gallon bucket for my 'takeoff pad' most of the time.... removed the metal bail (handle) and replaced with a rope. So I don't have any problems with metal in the ground or concrete anymore.