Grips vary and so do the recommendations. 1/8″ is probably general amount IF you want to take the grip cap into account. Some grips will have caps that 1/4″ would be more accurate if you are measuring the appropriate place on the grip cap. Personally, I trim the shaft to the exact length and do not account for the grip cap. In short, it depends on the grip as to how much to account for, and personal preference.