Data developed in the rifles would be subject to the variables present in the rifles, bore and groove diameter in particular.  My .301/.310 barreled rifle isn't going to produce exactly the same pressure and velocity as my .299/.308 barreled rifle when using .308 bullets.  We have no way of knowing the condition of the barrel on the 1898 that Nosler used to develop their data.  Start low, work up!  Also bear in mind that today's IMR powders are not produced in the same plants that they used to be, when considering older data.