Just me thinking, but I think you're way over-thinking this. Yeah, the B string is a whole bunch thicker than the G but if you're setting the spacing from string edge to string edge you're doing it by looks instead of feel. When you fret a string, you're pushing on the center of it and the center of it will contact the fret no matter what size string. Distance between them doesn't matter unless they're generally too close or far for comfort while plucking or picking. I don't know about all bass bridges, but I like the Hipshot A style and that one allows for string spacing adjustment so I assume many others do also.
For reference, I'm very happy with my last two builds and the spacing is as follows:
(4 String, BEAD tuning)
Nut width 1.550
String spacing at nut (centered) .410
Neck width at 12th fret 2.240
String spacing at bridge .715
If you draw that out you'll see that I have a generous amount of room between the string and the edge of the neck. Having some extra room there does not making playing difficult or uncomfortable, but it does allow plenty of room for error and prevents accidentally pulling the strings off the side of the neck. I can't see a reason to minimize the neck width to the point where pulling the strings off would be a concern.
Give it some room to breath, keep it simple and it will work perfectly well.