Hello Larry,
It is true that you don't have to accurately align the aircraft with magnetic north at the beginning of the magnetometer calibration. +/- 5 degs is fine.
You don't even have to perform this calibration on a compass rose, just a location where the earth's magnetic field is reasonably free of disturbances.
What is not true is that it doesn't matter how accurately you align the magnetometer with the centerline of the aircraft. This is one of the few "knowns" the calibration algorithm relies upon.
If you install the GMU 11 with an alignment error, that error will always be reflected in the heading displayed by the system.
A 0.5 deg error is insignificant, because heading is a relatively inaccurate measurement, and the TSO requirements for heading accuracy reflect that.
Even if you had a 1 degree alignment error, it is unlikely you would see that in flight, but anything more than a couple of degrees is likely to be detected at some point, especially if you visit a marked compass rose.
When we were first developing and flight testing the GMU 11, I installed one in the left wing of my homebuilt against the main spar believing this orientation would be perpendicular with the aircraft centerline. I built this plane 20 years ago, and had forgotten that the outer section of the main spar is swept back a few degrees on each side.
Each time we calibrated the GMU 11, it consistently reported a heading a few degrees left of the heading reported by the GMU 22 mounted in a different location in this plane. After looking at the raw data, we came to the conclusion that the unit had to be misaligned by the difference in headings.
Sure enough, when I referenced the main spar plans, I was reminded of this swept angle which was very well known to me at this point since it was within 1 deg. of the heading difference between the two magnetometers.
I built a wedge of the correct angle needed to correct for the sweep angle and aligned the GMU 11 perpendicular to the centerline. No more heading disagreement after that.
Thanks,
Steve