The measurement of the Girth itself is from each end of the buckles. You need about a 4-5" gap between the buckles for the nylon straps.
Example: say if your waist measures 40" around, then you would subtract 4" to get 36", so you could go with a 36" girth. If you subtract 5", you get 35, since there is no 35" girth, you would round up to 36".
Hope that makes sense. As far as being a 42" waist, I would go with the 38" girth.