Re: The ultimate lenght for a .308
jb100br is right you want your barrel as long as you can stand to get the best velocity out of it but if you feel the need to have a very short tube then a barrel as short as 18" would work , the bullets you have chosen will still be traveling plenty fast enough to be stabil and have enough thump for deer.
I shoot a 15" barreled Encore pistol in 308 , my load is 48grs of Varget and a 150gr Ballistic tip ,I get a velocity of
just under 2700fps with great accuracy
I have loaded some 168gr Bergers up to see what they would do and was able to get just over 2600fps with 47.5grs of Varget but the recoil was stout , but at that speed I believe that the 168gr Berger VLD would stay supersonic well past 600yds.
I have killed deer with the 150gr bullets out to 275yds+/-