The "general rules" and 'rules of thumb' for velocity loss are not very good guides. I've chopped a few barrels, and read many reports from people that have chronographed before and after a chop.
More often than not, even a 4" loss in barrel length is only worth 60-80 fps for many cartridges.
My hybrid/custom .444 Marlin, for example, is running faster with its 19" barrel, than most people get from the factory 20", 22", and even some 24" barrels.
My velocity loss from chopping 3 inches? .... Less than 40 fps.
My favorite accuracy load for my '06 rifle has a MV of 2603 with the 24 in. barrel.
It would drop to 2300 fps if I cut the barrel to 16 inches.
Which would put in in the class of my Saddle Ring carbine in 30-30.
Cut your Saddle Ring Carbine to 16" and they'll be back in their own classes, again.
I just chopped a Marlin 336 to 16.25" thinking I'd do a Marauder-esque build with it. The project went sideways and took another path, but I chronographed some loads before and after the barrel chop.
Overall, I only lost about 100 fps. One of the more extreme examples:
Win 170 gr factory load: 2252 fps @ 20"; 2110 fps @ 16".
I really don't have a problem with "slow" bullets in almost any cartridge, though. Most of my recent acquisitions and rifle builds have been 'big and slow', or 'big enough and slow'. (.30-30, .444 Marlin, .458 SOCOM [really slow], .475 Tremor [slight edge on the SOCOM], .35 Whelen, and in the non-rifle realm, .480 Ruger.)
As long as the bullet still has enough velocity for a clean kill, it's good enough for me. And if that comes paired with a rifle that's ridiculously short and nice to pack through the trees... even better!