Does the BCF roll come first or the decision to transfer damage come first?

It is always helpful to quote the power(s) in question:

Blades/Claws/Fangs
When this character is given a close combat action, you
may roll a d6 after making a successful attack. The result
replaces this character's damage value, then that damage
value is locked.


Mastermind
When this character would be dealt damage, you may
instead choose to have all the damage be dealt to a single
adjacent friendly character with a point value less than this
character's. Any additional effects of the damage dealt
(knock back, penetrating damage, etc.) are applied to the
character taking the damage. Damage dealt by this power is
not an attack.


B/C/F happens after the successful Attack. Mastermind Happens when the character would be dealt damage. The powers tell you the order. In order to be dealt damage the total damage must be calculated first.

A common mistake is thinking one must declare BCF before the attack when, if you read the power above, the decision to roll doesn't take place until after the attack roll occurs.