I’m preparing for PvP as I want to give it a try, mostly in Great League. I don’t know when exactly IVs are decisive and when there is a bigger margin. I got a 0/11/12 Kingdra (1480 CP). As I have read, when IVs matter they are decisive for the game. How much difference are 3-4 defense points under the recommended set? In what situations IVs are mandatory because the matchups are really close from each other or other reasons?
On coarse scales IVs matter. Your 0/11/12 Kingdra will fare somewhat better than a 15/15/15 Kingdra that is a few levels lower and has a CP near the cap. My guess is that a 0/13/13 would be a little better, but not much. On the other hand, a 15/0/0 would do somewhat worse (although not a ton worse).
On file scales, the IVs don’t matter much at all. My 0/15/15 Forretress comes in at 1491 CP. A 1/15/15 would come to exactly 1500, which would be a a very small amount better. In practice, both will almost always perform the same, in perhaps 1 battle in 2000, the 1/15/15 will get off one extra attack or win by a tiny bit instead of losing by a tiny bit.
IVs are largely for mirror matches when you’re up against the same Pokemon with equal shields. If your IVs a slightly less than optimal, it’s not a guaranteed loss. You both need to play perfectly for the IVs to be the deciding factor.
Even with slightly imperfect play, the mirror match usually ends with both pokemon firing off charged moves at the same time, for a tie, at least in simulations. Now the new rules will have the higher attack pokemon going first, and hence winning. So it’s now like e-bay bidding, a single point of attack over the foe matters a lot.
That said, this dichotomy plays out in a small percentage of all matches, mirror or not. Most of the time, the big picture factors (species, moves) matter way more. (I’m assuming the pokemon is fully leveled for the league it’s in, so that doesn’t apply anymore.)