Gregor Clegane question

By Crifmer, in Battles of Westeros

A question about Gregor Clegane's (The Mountain's) special ability:

His normal ability says that when you roll a flag, add an additional die for each flag rolled. Any flags rolled on these bonus dice do not add additional dice. This ability by itself makes perfect sense. The question I have is in regards to flanking:

Do you add additional dice before or after the flanking reroll? If you add them before the reroll, do any new flags add dice? Do you have to reroll the bonus dice separately so that if those roll flags, they don't add new dice? If you add them after the reroll, then do you not get to reroll those dice, if they initially roll the symbol you chose to reroll? This situation gets a little sticky, and some clarification would be nice.

Thanks.

The way we played it after reading through the rules a few times to get the wording correct.

You roll the attackdice as normal and then for each flag you roll an extra dice, after that you do the flanking reroll where you choose one symbol and reroll all dices of that symbol. No extra dice for flags this time around.

That's the way we've been playing it, too. I'd just like to see what the devs intended (if that is actually the right way to do it).