BTW, the Rick's way is the way many manufacturers measure their packs.(some use ping pong balls)
Because most packs have very irregular shapes it is pretty much impossible doing it another way.
As far as using the given measurements (manufacturers specs) take those with a grain of salt because they can be rounded off (IE 50/60/70 L) or simply include the theoretically possible volume of the pockets.
Theory and practice here differ because in most cases you cannot get the same volume in those pockets when when the pack is also full compared to when it's empty...
You could always make several 5-10 L bags filled with those polystyrene balls and take those to the shop and see if they fit inside a pack.