OK. Issues with mag clasps interfering with compass. As a physicist (and computer scientist) I had a theory and I’ve just done some experiments which, although not peer-reviewed, seem to me to tentatively support my theory.
My theory, very tentatively confirmed, is that how much a mag clasp interferes with the compass is related to wrist size vs strap length which essentially if sticking to Apple branded straps reduces to only being related to wrist size and whether you chose a 40 or 44mm watch. (Strap length will be constant with only two states, whether you chose a 40mm or 44mm option, so once that choice is factored in then the only variable is wrist size.)
So how does wrist size determine the degree of compass interference? Well, the point of minimum interference is if your wrist is the “perfect” size for the strap whereby the magnetic clasp is lying directly below the face of the watch (along the perpendicular to the base of the watch). At that point the magnetic interference is symmetrical around the radial axis of the watch (and I am assuming hence is also axially symmetrical to the compass sensor on the assumption that the compass sensor is aligned parallel to the watch face) so the net distortions cancel out. If however you have an unusually small wrist so that the mag clasp wraps right round and is closer to the top edge of the watch, or alternatively you have a very large wrist so the mag clasp is closer to the bottom edge of the watch (assuming left-wrist wear with right-handed orientatio), you will get asymmetric mag distortion of the compass sensor which will cause bad effects.
I have small wrists so my strap almost doubles around my wrist and the mag clasp is very close to the top edge of my watch and I get bad compass errors. When I artificially slid the clasp round to the perfect half-way opposite-the-base-of-the-watch position however, and I kept the watch on my wrist by pulling the slack in the strap tight with my fingers, I got no compass distortion.
For anyone worried about compass distortion I suggest trying the strap on in store and if it sits near the half way point you are probably OK, it it fits at either the biggest or smallest extreme of the strap size then you probably will have compass distortion problems.
Equations available on request. (No, not really. Go and ask Maxwell 🙂)