Hector said:
I don't think the kaioken level you can handle is linear to the power level.
There's clearly some form of relation between power and the Kaioken level you can handle. Goku couldn't even dream of handling Kaioken prior to training in the Saiyan arc, and he could only comfortably handle 2x and wrecked his body going above it after training. After his power rose to 90,000, he said his body could handle 10x, which is likely the level he was referring to when he said he could raise his power further than the level he showed Ginyu in instantaneous bursts. Against Freeza, he could now handle 10x throughout an entire fight, and seemed to sustain less damage from 20x than when he fucked himself over with 4x in the Saiyan arc.
Obviously Goku wasn't only 5x stronger against Freeza than when he fought Vegeta because the maximum level he could attempt only rose 5-fold, but his tolerance of higher Kaioken levels increased as he trained and got more Zenkais.