BC Rider: the point you make about having some precise way of measuring the set produced for each side is a good one. A stick taped to the down tube and seat tube would work; wish I'd thought of it in time.

That done, one could try the symmetrical, axle method and then use single-side loading to correct a difference if one was detected.

Symmetry isn't the only benefit of the axle method - it also loads in a very controlled way with a perfectly-aligned force that engages an intended load-bearing face.

I must say I'm bemused by the concern for precision. The spreading amounts to 1/5 of an inch - in a system where the wheel is mounted by eyeballing for straightness, then clamping with a friction device. I end up adjusting the lateral position of my brakes - because the wheel has moved enough to cause dragging, a matter of millimeters - as often as not after putting the wheel on. If that level of imprecision mattered, we'd be screwing locator bolts through axle-mounted ears into holes on the dropouts - and getting engine-type tolerances.