Question on ballistic app data inputs

[jackwagon]

Hey guys, I downloaded the Strelok app and was starting to input my data when I realized the 2580 fps for hornady black 123 gr is for a 24'' barrel, I am shooting an 18''.
So the question is. Without a chrono is there a formula or base reduction for different barrel lengths?
or anyone else with an 18'' that has used hornady black and has chrono data. I am just looking to get close.
thanks.