I think I used a 120 mm width. But it's more a matter of getting the chainline right, so the BB width required will depend on the cranks selected and the desired chainline. Me, I had to cold-set the rear triangle to 135 mm to fit a drum-brake SA 8-speed IGH, so the chainline was pretty wide. Measure the distance to the sprocket from the dropout, subtract that distance from half of the dropout O.L.D.- that's the chainline. Then somehow measure how far the chainring is from the end of the axle when looking from above when the crank is installed, add that distance to the chainline measurement, then double it- that will be your BB width. Make sure to use the same taper on the axle you measure with and the BB you eventually buy- Shimano is JIS taper, of course.

-Cycle_Maven
(used to be known in these parts as Squeazel)