For pure accuracy sake, the best barrel length is one where the tightest spot in the barrel is at the muzzle. The way to determine this, either in a blank or finished barrel, is to remove a lead bullet from a round and push the slug down the barrel with a rod and feel for the spot of most resistance. So long as the tightest spot is at a legal length, mark the spot and cut and crown there. This is how custom gunsmiths who build the best shooting match rifles determine the length of their finished barrels. I've slugged a few custom match barrels and it's not unusual to feel loose spots in the barrel where there is little resistance. You don't want those loose spots at the muzzle.
