The IRO MTB hub has a 52mm chainline, and the track hub has a 42mm chainline, according to their website. So:
Measure the distance from the seat tube to the center of your current chain ring. (x)
Measure the diameter of your seat tube. (y)
x + (y/2) = current crank chainline.
Subtract hub chainline from your current crank chainline.
If the result is negative, increase BB width by twice the result.
If the result is positive, decrease BB width by twice the result.
Take off cranks, measure BB width.
Buy the closest size BB to the calculated size.
Seat tube to chain ring = 30mm
Seat tube diameter = 31.8mm
Chain line = 30 + 15.9 = 45.9mm
45.9 - 42 (track hub) = 3.9mm offset
Current BB width = 120mm
120 - 7.8 = 112.2
You need a 112.2mm BB, get the closest size you can find, probably 113. The resulting chainline error will be less than 1/2 mm.