In SSPBI the defender will not get a chance to influence the countdown. This means there is no to and fro over wanting to reach the game end.
In most (not all) cases the attacker will want more time in order to put his plan into action.
This mans the countdown will always be toward the lower level of increment.
Therefore some random is needed.
I propose A D6 with re-roll allowed.
Here is the thinking.
Average score on a D6 is 3.5. This means that a 21 point countdown gives 6 turns.
If the player is seeking a longer game the average will be nearer 2.5.
6 x 2.5 = 15. Trouble is that the occasional 6 will still happen and "chew up" a lot of the game length.
The countdown will start at 17 for the first play test version.
The problem here might be that the attacker will get the first and last turn.
We could also penalise the attacker in terms of victory points for every extension?
Randomisation is also good - we don't want any cast-iron certainty here!
Indeed