Assuming no one has a great hand like pocket Q’s, 9’s, or 2’s, or two pair, you have nine outs to the flush and 3 more for top pair and even a remote runner, runner straight or straight flush possibility which isn’t worth an out.

If you’re pretty sure you won’t get re-raised or see heavy action on the turn or flop, I’d bet somewhere less than the 45% of the pot, which is the odds you’ll get your hand if you see it to the river. The bet might win you the hand right there since all but one person has showed weakness and you’ll have position on all of them except one for the rest of the hand. If you'd rather play it safe for fear that one of those premium hands are out there, bet less than 35% of the pot to maintain the proper odds (9 outs instead of 12).

That’s a basic approach, but there’s more to consider like the personality of all players – especially the one who has yet to act, the action pre-flop, your image, and so on.