I.e., Do you:
- Roll reinforcement die, and add troops to make the short fall?
- Declare the number of troops you're using, and use the reinforcement die in an attempt to bolster your numbers?
The rules say:
Roll the die first; select the Region you wish to conquer; then place the required number of Race tokens (minus the die results) there.
Since you roll the die even before you decide what to attack, it would seem very unlikely that you'd need to determine the number of attackers beforehand. This is further bolstered by a thread at the BoardGameGeek forums where a lot of Small World rules clarifications posted by the game developers are gathered. This is what it says about Berserk:
You may roll the die once per attack. First, you roll the die. Then you choose an adjacent territory. Then, you deploy your tokens. (emphasis added - ErikP)
About the final roll, there is a very important sentence in the rulebook, in the "Berserk" section: "If you do not have enough tokens left, this is your final conquest attempt for the turn." This sentence means that there is no "end-turn" die. For each conquest you roll the die and select your region: if you do not have enough tokens left, then your turn is over. You are not allowed to re-roll the die.
The reinforcement die with Berserk provides a "discount" on the number required to take a given territory. It's just like a reinforcement roll at the end of a regular turn. That means you must be able to take the territory with your available troops plus 3.
Example:
You've got 5 counters and you want to take a territory that's got 2 counters on it and a mountain. That's 5: - 2 for any space - 2 for the counters - 1 for the mountain
You roll 2 reinforcements. You take the territory with 3 counters. That leaves you 2 counters. If you now wanted to take an identical territory, it would be your final attack, because you would now be making an ordinary reinforcement roll at the end of your turn - because you couldn't take the territory if the reinforcements fail you.