Salty nuts recently had a PB alpha rejected by his captain because of degraded data, so he's been experimenting with 3 GT31s to find the best way to get reliable results. I told him the best was on his head, but he hates helmets, and viewed that as a last resort.
So he's tried on his back, his shoulder and each upper arm.
The one on his back gives the worst results, I guess because there's a big change in sky view as you lean over,.
The ones on each arm gave some good results and some not so good, strangely the fastest alphas had the most degradation, possibly because his gybes were more aggressive?
The one on his shoulder gave the most consistent results.
And happy to say, he's just beaten the rejected PB by around half a knot, with very good data.

And of course as Sailquick says, for very best results, you can't beat wearing it on your head, there's less bodily shadowing, and movement up there, the head is designed to keep as steady as possible for accurate vision.