There are only two realistic factors for accuracy (or not).
What you're looking at is a piston in a cylinder exactly 1.00 square inch in area, and a pressure gauge. Both have to be accurate and if they are, the device is accurate.

As it is much easier to produce a cylinder that is 1.000 square inches, the major source of inaccuracy is the pressure gauge. SO, if the pressure gauge is accurate to within 1%, that is what the device's accuracy is, as well. If one wanted to be idiotic, the distance the centerline of the gauge is above or below the centerline of the cylinder counts too, but it is usually much smaller than the gauge's inaccuracy.

IMHO it is much easier to be inaccurate about measuring the distance between the vice jaws than any other part of this process. If one wishes to be super accurate buy a super accurate gauge and have it calibrated by a deadweight tester traceable to the National Institute of Science and Technology, before the series of spring tests.

There are other less accurate ways to assure accuracy.

R.