Where is the betting margin?

[VivaLaViking]

Player Hero acts first, he is certain player Villian has a 4-flush after the flop. Hero knows he will win unless the next suited card falls. Hero knows Villian has 9 outs and stands at 35% of making the flush in the next two cards, at ~19% on the tun and at ~19% on the river. How much should Hero bet on the flop to make Villian's call incorrect.

Hero considers that the bet, B, should be > 35% of the pot and feels that is where the margin is. But then he considers that Villian only has a ~19% chace of making the hand on the next card so the proper bet would be > 19% of the pot. What should hero bet, more than 35% of the pot, more than 19% of the pot, or some other percent of the pot ?