In WoW, I used the following general rules with great success:
(1) Bosses are always 1.0 and function as a baseline.
(2) “Big Adds” are either 1.0 (if equal prio to boss) or 1.2 (if considered high priority).
(3) “Tiny Adds” are either 1.0 / (# of adds in a pack) or 1.2 / (# of adds in a pack) (if high priority).
(4) Fights can have special rules as needed where adds are worth 0 during some specific buffs or phases.
So mainly I just need to know for each fight which adds fall into the (2) and (3) categories and how many there are per little pack for (3).
And then of course I’d need to know (4). An example of (4) would be not DPSing those irrelevant balls on A6S.