Remember that the strings spread out as they run from the nut to the bridge, so the bridge saddle spacing will always be a bit wider than the pickup polepiece spacing.
You’d probably be best working back from the bridge and nut design widths for your neck as you’d normally have the outer strings running parallel to the edges of the neck, say 2mm from each edge. You could draw this out, either physically or on a drawing program, and then measure the string spacing at the pickup position to double check. But also remember that a certain Leo Fender had a small hand in designing the Stingray, and whilst not having a hands-on role, I’m sure he wouldn’t have let them vary too much from the Fender dimensions.
Also, the MM pickup has very wide pole pieces, so the string spacing can vary quite a bit and the strings still run over the polepieces.