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When is the precise moment i should be rolling on the chart? For example is it as soon as i place down the unit during deployment or wait until we are about to start turn 1 etc?
I don't know about everyone else, but I have always rolled for this at the same time as determining spells. However, I see your confusion, as the rule book only specifies "at the start of the battle," and BRB never gives (to the best of my knowledge) a clear definition as to when the battle actually starts: when you are rolling for spells, when you are deploying your units, or at the beginning of the first player turn of the game. That being said, I doubt you will find anyone who will argue against you rolling for this when you generate spells, as it allows both players to be better informed about what you have when you are deploying.
This is how I do it. How about everyone else?
Well Papa I would do it as you say.. Before deployment...
That gives the advantage to your opponent so they have the heads up. No one could complain about that!