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%
That is not how quicken spells work. Only the cost for Twinned spell scales as you describe. In 5E, quicken always costs 2, regardless of level. Larian increased this to 3, which is fair considering how powerful it can be.
And of course damage calculations assume that the blasts hit. That’s obvious and does not need to be stated.
