It’s about 25fps give or take for most cartridges ever inch between about 18 and 28 inches. Hit the extremes and it might be more of less. Also depends on cartridge and barrel specifics, but 25fps is the number most people use to estimate loss. It has held true for me.I had my 24" barrel cut to 20" because I wanted to use my suppressor.
Using the same box of factor ammo using the 140g Nosler BT. I lost 100fps.
I am glad I went with the 20" barrel.
A buddy has a 20” 280ai and it does not need to be longer, IMO for a hunting rifle.