@Fifty7 For this kind of flight my advice is switch to ATTI before motorStart and leave it there. That's what I'd do. Whatever you do, DON'T RTH and disable Smart RTH so that the A/C doesn't initiate RTH on it's own.
It's not just distance to the ferrous material that matters. It's also the location of the A/C relative to the ferrous material and the direction of the geomagnetic flux. It's not just magnitude. The ferrous material causes a distortion of the geomagnetic flux. An analogy is trying to cross a smooth river in a straight line. A constant thrust angle will get you from one side to the other in a straight line. Throw in a few large boulders, the flow becomes distorted and the thrust angle depends on your location in the stream. But, it's not just the proximity to the boulders.
Here is a flight where it wasn't just distance.The altitude was 30 meters when a geomagnetic distortion was encountered.
The direction computed from the magnetometers became different from what the gyro was saying and a compass error was declared (not shown is the switch to ATTI)
This all happened in a location very close to the previous path of the P3 with no adverse effects
Many, if not most, of the compass error flyaways occur because the gyros have said one thing and the magnetometers have said another. But, the problem can be that the FC's idea of heading can now be incorrect and an RTH will cause the A/C to fly in the wrong direction.
On the question concerning the use of the compass in ATTI mode I agree with
@Vertigo,,sort of. In the first place the magnetometers aren't the primary source of heading info. After motorStart it's mostly the gyros. But, the real question is, without GPS info what would any kind of heading (gyros or magnetometers) info be used for. The only situation where heading info is used in ATTI mode is in an RTH in which case the P3 will fly the gyros, not the magnetometers