Basic flush draw question

[Peleus]

Hi all, I don't think this is anywhere near complicated enough for the probability forum, so I'll post here to confirm what I'm thinking.

When at the table, I use the approximation of outs x 2 for the turn to bring what I need, and x4 for the turn or river.

As such on a flush draw (2 on flop, 2 in hole) I have 9 outs, so 18% chance of making the flush on the turn, or 36% of making it by the river.

What pot odds do I need to call however? Do I base it around making the flush by the river, or do I base it around making the flush on the next card (smaller %).

I beliveve it would be based on the next card, as the turn would be other unknowns such as another bet, can someone correct me if I'm wrong?

Cheers.