Most of the parts will tolerate a fair amount of variance in length.
When you do your layout, mark one end and one face of each piece and reference all of your measurements from these points. This will keep the relative position of all of the holes accurate from part to part - which is critical. You do not want to be making measurements from both ends of parts that are not identical in length!
Bob - now that I've written that down I am reminded that having running dimensions from a common reference point on each part would be a nice upgrade to the plans.