Larian have changed the rules on quicken spells it should cost 1 point per level of spell and 1 point for cantrips; that make 2 EB very cheap, especially at high levels. As it stands you are better off twinning 2 fireballs most of the time (or 3 with haste)

btw the damage calculation assumes all the blasts hit, I usually calculate hit rate at about 65/75%