Okay, so each splitting of fireball costs 1 Firepower (base of zero)?
Since splitting the fireball splits the damage dice between them I wouldn't have it cost Firepower since it doesn't increase the total damage. Indeed, splitting a 2X damage fireball into two X damage fireballs would do half the damage to each target per fireball.
Instead, I'd have a standard low-powered fireball that costs a token amount of Firepower and a high-powered fireball that does 6d6 more damage. I picked 6d6 since it neatly divided into a single +6d6 fireball, two +3d6 fireballs or three +2d6 fireballs.
Speaking of multiple fireballs, if a target is covered by the radius of more than one of the fireballs in the burst I was thinking about adding the damage together and have it make one Dexterity save to half that total. It'd be less fiddly than having it saving for each fireball separately.
However upon reflection, multiple saves would justify a slight Firepower cost for splitting the fireballs as it'd make it harder for the target to avoid damage.
So if the
Multiattack averages 162 DPR we want the Fireballs to be around half that at 81.
Which suggests a low-powered attack of one 63 (18d6) fireball, two 31 (9d6) balls or three 21 (6d6) balls and the high-powered attack of 84 (24d6), two 42 (8d6) or three 28 (8d6).
Hmm… perhaps we should call it something different than Fireball. Firevolley, Firepeppers, Fireburst, Firestorm, or whatever.
One of those names was not a serious suggestion.