PFD Isn't Level When You Set Up INS13 for GPS Flight
When setting up a new GPS quad it doesn't look level in the PFD/HUD. Here is the reason and a calibration procedure that fixes this. This procedure assumes that you are using aux mag only "AuxOnly" (not OnboardOnly or Both). Actually, it will probably work with "Both", but I haven't tested that.
I think it's a good idea to have Attitude mode on the flight mode switch for use in emergencies. If you use Attitude mode and adjust this leveling issue with just "Rotate Virtual" and then switch from INS13 back to Basic (or Basic to INS13), it will drift. Basic needs one trim and INS13 needs a different trim. The adjustment discussed here will make it level whether you are using INS13 or Basic, without recalibrating each time you change INS13/Basic.
The Reason It Isn't Level
When using Basic Attitude Estimation Algorithm (Configuration->Attitude->Settings->AttitudeEstimationAlgorithm), otherwise known as Complementary Filter (this is the version used by CC3D), you are only using the accels to tell you what is level. When you switch to INS13, you are using the mags as an additional source of leveling information. The direction of the mags changes according to your location on earth and is approximated by the use of the World Magnetic Model in the flight code. For instance, where I am in the USA, the mags point about 62 degrees down. Close to the equator, the mags point about zero degrees down. The accels say "down is this way", and when you add the mags, the combination of accels and mags says that down is in a little different direction.
The Calibration Procedure
Use Basic Attitude Estimation Algorithm (Configuration->Attitude->Settings->AttitudeEstimationAlgorithm) with Attitude mode and adjust Rotate Virtual (Configuration->Attitude->Settings->RotateVirtual) for a motionless hover. If it is drifting forward, you subtract from pitch. If it is drifting left you subtract from roll. Etc.
Then use INS13 Attitude Estimation Algorithm with Attitude mode and adjust aux mag orientation (Configuration->Attitude->Magnetometer->AuxMagOrientation) for a motionless hover. If it is drifting forward, you subtract from pitch. If it is drifting left you subtract from roll. Etc.
Notes
Your blue "Aux Mag Orientation Help" bars won't be all zero any more. That's not a problem. In the real world sense, the aux mag is now aligned more accurately than the onboard mag.
The OnBoard mag doesn't have an adjustment, so this only works for AuxOnly. It probably works for "Both", but that isn't optimal.
To avoid this issue, the World Magnetic Model would have to be perfect, the mags and accels would have to be calibrated perfectly, and the aux mag would have to be perfectly aligned to the model. Even with all that, there are some models (e.g. helis and tricopters) that are not level when they hover, even if built perfectly and those would still have this leveling issue.