The answer that you're seeking probably doesn't exist. It probably doesn't exist because how much velocity you get isn't just influenced by barrel length alone. The bore diminsions, rifling style, chamber diminsions, leade or freebore, and the smoothness of the bore finish all influence velocity, too. None of these diminsions are exact, but are made within a range of tolerance. So, you can have two 20" 336's shooting the same load from the same lot and if you expect them to both chronograph the same, you could be very surprised when they don't if you didn't consider everything else that could influence velocity from one rifle to the next.
You can also have a "fast" short barrel come very close to duplicating the velocity achieved by a "slow" longer one.
There are just too many variables involved. Which is why you get "general rule" type answers to the question of how much velocity difference you can expect between a barrel of x length and one of y. And in general, a shorter tube gets less velocity with more muzzle blast than a longer one does, as you already know.
I've typically got higher velocity from my 336 from a given load than I've seen chronographing the same load out of Winchester M-94's. It's never been enough for an animal getting shot to fret over the difference. I have assumed that Marlins' use of the Microgrove reuslts in a more effecient gas seal, a smoother bore, and less friction due to shallower lands. Then again, the difference could be due to my rifle having a fairly tight or minimum diminsion chamber while the chambers on the Winchesters I've chronographed were far more generous (my fired cases fint in them but their didn't fit in mine). A smaller combustion chamber is like higehr compression in a car engine in the sense that you get more power from the resulting bang.
So, I think the answer that the original poster is looking for may not exist, but I'd like to see it if it does!