You really need to be able to measure the chainline to know what BB you need. If you have calipers, it make it very easy. Measure the diameter of the seattube and divide by half. Then measure from the side of the seattube to the centerline of the chainring and add that to the first number. For example, my seattube is 29mm/2 = 14.5 + 26mm (the distance from the seattube to the chainring centerline is) = 41.5-42mm chainline on the cranks. If yours adds up to 46mm and your hub chainline is 42mm and you have a 115mm BB spindle, you'll need a 107mm BB (8mm shorter spindle will shorten each side 4mm. Was that comprehensible?