A BA0 bolt has is 6mm in diameter, 1mm in pitch and needs a 10.5mm spanner.
From here, multiply the pitch by 0.9 to get the pitch of the 1BA screw, and do that again to get the 2BA screw pitch.
A website called sizes.com is kindly hosting the original discussion papers that lead to the standard, and what thorough documents they are.
The committee was trying to take over where Whitworth left off, at 1/4inch, using mathematical progressions to create progressively small screws that would have an optimal combination of thread and core size to maintain strength as they shrunk.
They compared their theoretical screws with real non-standard screws of similar sizes (as Whitworth had done before) to ensure they were tracking industrial practice.
Not wanting to reinvent the wheel, it appears, they leant heavily on the work of Swiss academic Professor Thury, who had done very similar work a few years earlier in defining a thread system for Swiss watch makers – Thury’s system actually continued beyond 6mm to huge bolts, the largest of which was 75mm in diameter.
The BA committee also thought deeply and realistically about borrowing a metric system for use in an Imperial country.
Get a cup of tea and spend 10 minutes reading these three thoughtful documents:
2nd BA paper (look out for the word ‘aliquot’)
Hats off to sizes.com