Just FYI, one can tell by these Joe Bardens, and other staggered pickups, that it needs just to be a miniscule amount of millimeters or hundredths of an inch to produce difference in timbre, and output. Hence that these arcs of the rails are like just one and a half millimeter higher in the middle. I think Bartolini once had measures (on his site) how much lower in output a pickup will become just by lowering it the slightest. It was like if you lowered it by the thinnest string gauge of the g-string (say .040) the output were cut in half. In db. Or something like that. That measurement was valid for most of his rail/ceramic pickups.
So the radius of both strings and pickups makes the most sense, here it seems.
So the radius of both strings and pickups makes the most sense, here it seems.