Need help with pot odds

Hi there. I've been playing for a while and thought I had a fair understanding of pot odds - but something is troubling me...this may sound stupid

It may be best illustrated with an example.

Say I hold a four flush after the flop.
I know that the odds of hitting my final flush card are 19% with one card to come, and 35% for two to come.
Now as I say, I have ONLY seen the flop and I'm deciding whether it is correct to call to my draw.
The bet to me is $10 for a pot of $25 (2.5:1)
Does my choice in calling to see the river then take into account the odds of one or two cards to come (19% or 35%) ? For some reason I thought that I must work it out on a per card basis....therefore making sure the pot is 5 times the bet in order to see the river card, AND then making sure the pot is 5 times the bet to see the river.

I'm really confised

You can't be assured that after calling flop bet (that is in your example half pot bet), the opponent will not bet the turn if you miss. When you face flop bet and consider a call in headsup situation, you are 19% to hit - facing 1/2 pot gives bad odds here. The exception is whens the flop bet puts you or your opponent all-In, then you are sure that you will see the river without any further expenses, then you use 35% when you consider calling.

More tricky play is to RAISE flushdraw when you are in position or bet it out if you are out of position - then you hope your opponent will fold and if he calls he will check the turn allowing you to check behind.