Are the dimensions of SD humbucker covers some kind of top secret?

[David Schwab]

Another traditional method is to use a vernier caliper with both inside and outside jaws to measure the distance between outside edges of the outer holes, and then the inside edges of the same holes, and take the average of the two measurements. This is quicker than two diameters and a distance, and more reliable than eyeballing alignment with two left edges.

Yes, that would have been the way to go if the screws or slugs had been absent. Clearly I didn't eyeball them very well the first time!