would part of the difference be the pump itself? most pumps are rated for cfm's based off 15" of vacuum, but i will say i have been using that as a guide and then increasing to the next size and the impact has been incredible, but also i run a lot of flat ground so i generally need a larger pipe to do the same work.
I agree it never hurts to update or re-study, but i wonder if it would make more sense to develop an online calculator for people to use for sizing mainlines/wet dry's.
7000 taps on vacuum, just trying to get a little better every year.