In my opinion, it would be best to have heavy barrage available for:
1.Attacking BG, as soon as the battle starts ... but with a 2 minute countdown timer. Then the attacking player would need to decide where the heavy barrage will land within a short period of time ... and if timer expires before the barrage is used, then it can't be used during the remainder of the battle.
I think this is how many WW2 heavy artillery was used ... first the barrage at a specific location, then the attacking units advanced. I've read that the German's used this predictability to their advantage, by placing more valuable assets (tanks) further back ... and then moving them forward after the Allied barrage hit and the location of Allied penetration was determined.
This may help CC too ... so that map defenders may want to place units a little further back until timer has expired ... to minimize losses. This would also allow attacking force time to shake out of the deployment zone without being zeroed in on (with too many predetermined lines of fire) ... at the start of a battle.
2.And map Defender ... the heavy barrage would become available after a few minutes. This simulates the time to communicate and organize, and allows attacker time to shake out of deployment area.
So, in other words, two methods of timing the heavy barrage ... one method for the Attacker and another for the Player Defending.
Good sense in that David. Maybe the attacker could have a 50% chance of getting the barrage again from half way to the end of the allocated time - this is also another touch of realism since you "booked in" a barrage as part of an attack/battle plan and then if you did not use it you had to compete with other commanders' priorities.
Hard work is a good way to succeed. There is less competition that way...