You'll find the fps/grain between two points is actually quite linear provided the maximum load is not compressed and the charge doesn't cross a pressure where case expansion differences affect linearity (usually for loads that peak around the 30,000 fps range). It's actually more linear than KE is, though both are pretty linear. Once you start compressing charges, though, the air space in the case between grains is changed, and that can cause non-linear changes in pressure.
For load data and commercial ammo stated velocities, these are, as mentioned, taken from standard pressure and velocity barrels. Under SAAMI standards the lengths of these are 24" for the majority of rifles (with a few exceptions, like .30 Carbine at 18"), and for handgun rounds they vary a good bit more, sometimes having two standard lengths for a single round. The SAAMI standards are available on line, and pressure and velocity test barrel specifications are in the second halves of each of these documents:
Rifle
Handgun
Rimfire
Shotgun