That doesn’t quite work past their initial bounties.
Luffys 100 - Zoro 60 (60 percent)
Luffys 300 - Zoro 120 (40 percent)
Luffys 400 - Zoro 120 (30 percent)
Luffys 500 - Zoro 320 (64 percent)
Luffys 1.5 - Zoro 320 (20 percent)
Where as Sanjis has fluctuated and now Sanjis bounty actually surpasses Zoros. There’s no mathematical average.
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.
Only exception
Enies Lobby
Zoro 120M - Luffy 300M
= 40%
Special circumstances
Marineford (No strawhats except Luffy participated)
WCI (Zoro didn't participate)