Some manufacturers are pretty close to 'average' with their advertised velocities. Some are way off, because they're advertising their best numbers from a test barrel.
In my experience, Nosler and Federal tend to hype the numbers. And most "hyper" or "ultra" velocity branding is going to be optimistic. Hornady Superformance is one that fits there.
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Your load will change about 25 fps per inch of the barrel length.
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This "rule of thumb" is never right.
I have never seen this be even remotely correct for barrels that I have cut.
Is it a reasonable baseline for expectations when cutting a barrel?
Maybe. For 'standard length' barrels, shooting certain cartridges.
But it is never right on the money, and has no chance of being realistic if the barrel is unusually long, or starting 'short'.
If you want to know what the muzzle velocity is from your barrel, you need to buy a chronograph or find someone that will let you use theirs.
I have had many experiences like jmr40's.
In one 'outlier' case, I maintained the same velocity from a .270 Win barrel after cutting it from 22 to 19 inches. (Same ammo, before and after. Six loads tested, I believe.)