Bounty is not strictly about power level true, but since the major factors are resumed by simply stating: level of threat, then of course if he kills a yonko he will most likely be considered a huge global threat, in particular if he will have contributed a lot in the demise.
He got an increase of 200 Million to 320 Million basically for cutting up a mountain and a Haki User. And for being the right Hand man of Dressrosa's savior I suppose. CP-0 is also present in this Arc so everything he has done is likely to Be taken into consideration. Cutting Kaido, Helping Defeat him in a direct confrontation and likely cutting down the mighty King will at least make him jump up to like 2 billion. Heck even 3 billion is possible IMO.
I will be extensively pissed if Zoro doesn't get at least 1.5 to eventually 2 billions assuming that he will indeed return to the rooftop or anywhere the fight will play out in the moment and ultimate the job in a consistent way.
You're omitting what I wrote... I'll insert it below once more. But first elaboration
There is only one (1) exception (Enies Lobby when Luffy after Gear 2 and 3 pulled away from Zoro). And even that one could in part be explained by the Strawhats/Luffy declared war against the WG. That act in itself might have given Luffy, the captain, 100M boost and his bounty might otherwise have been 200M which would have kept the ratio at 60%. You could say kind of how Kids bounty was 315 because of the acts rather than strength where as Law "only" had 200m before skyrocketing after Rocky Port and the 100 hearts.
Then there are two special circumstances - i.e. arcs where Zoro didn't participate where as Luffy did (Marineford & WCI). The opposite would be true if say Zoro, Sanji or another SH where sent on a mission alone and did something remarkable.
Zoro 120M - Luffy 300M
Marineford (No strawhats except Luffy participated)
WCI (Zoro didn't participate)