According to Shimano documentation for frame specifications, the spacing for a double is around 8mm.
On a triple, inner to middle is around 8mm, and middle to outer is around 7mm.

It looks like they measure to the inside surface of the chainrings, so if we add 1mm to the dimensions, then an inner ring on a triple is at around 37mm, and inner ring on a double is around 39mm.

There was another page somewhere that said braze-on is between 24.5-25.5mm.

http://pictoview.shimanoweb.com/eboo...ts%2027-96.pdf