If I'm building a 7 Rem Mag or 300 Win Mag I am going to spec a 26" barrel so all that Retumbo or H-1000 gets burned and lets the magnum be a magnum.
Not 100% sure but the 20-25 fps rule of thumb does apply to short actions. It may be more significant in the 7 and the 300, due to the slow burning powder they use.
On the other hand, the shorter the barrel, the stiffer the barrel. The shorter the barrel the more velocity loss, but it will be less picky on "the" powder charge for a given bullet.
I know, I'm no help at all.
That's why barrel tuners like the Winchester/Browning BOSS system should be much more common. Instead of going through an elaborate, time-consuming and expensive load development regimen, you tune your barrel to the load instead of the other way around.
It makes more sense and saves you money. Another bonus is that you can tune up for a common factory load and have settings recorded in case you lose your reloads on a hunting trip and have to buy factory stuff.
There are a number of ways to make tuners that are not covered by the Winchester/Browning patent. Take a look at .22lr match guns and you'll see a variety of systems. - What it basically involves is a movable weight near or at the end of the barrel that has markings for repeatable settings, which will change the barrel harmonics to suit your load.
It doesn't have to be real big, heavy or tacky-looking, but it can.
The simplest tuner is a rubber do-dad that you slide up or down the barrel to find the sweet spot, and friction holds it in place. It's pretty tacky but it works.