Plane ACC calibration. Dev question

I have asked this question before, and a lot of people still have different approaches, but I want to ask it again in light of this observation below. I generally do not do anything other than a one axis level calibration now as I am not flying acrobatic planes. @tridge has told me this is ok but does recommend a proper 3 axis calibration.

I have started using a nice $2 App that gives me pitch and roll readings. I place this on the level part of the fuselage like pictures. Since the Mini Talon likes flying with a 3 degree positive attitude I position as such and do a level calibration.

I then decided to do bit more testing. My phone shows 5 degrees of roll while Mission Planner is showing 7-8 degrees so the Matek IMU does not seem not very accurate. Also there is a lot more drift with the Matek 765 Wing accelerometers than my phone. With the plane staying absolutely still I noted it went from 0.17 degrees of roll to 0.25 degrees. My phone seemed more stable although it is only displaying 1 decimal place.

Would a 3 axis calibration yield a more accurate pitch and roll angle reading? It is such a PITA trying to do one properly.