Barrel Length's Effect on .45 ACP Bullet Energy

I was just curious if anybody here knows a formula for figuring out how barrel length affects a round's energy (ft-lbs.). I'm interested in getting Para-Ordinance's "Carry 12," that has a 3.5" barrel, and I wondered how much energy a barrel 1.5" shorter than a standard 1911's will take off a bullet?

For instance, Double Tap has a 185gr. round that, from your standard 5" 1911, has these figures - 1225 fps, 616 ft-lbs. Does anyone know a formula I can use to estimate how a 3.5" barrel will affect both fps & ft-lbs, given this example?

Thanks