Yes, if someone bets half pot you are getting odds of 3:1, or 25% from direct pot odds
. As for your question, this is where it gets a bit more complicated. There is another thing called implied odds, which basically means that if you do make your flush, you are likely to get paid more than if you don't hit it, and you also have to factor in that your opponent may or may not barrel on the turn. At this point, it comes down to your estimation of what type of opponent you are playing and how likely they are to pay you. I would very rarely fold a flush draw on the flop to a 1/2 pot bet because you are getting a great price and you almost have good enough direct odds if they don't always barrel the turn, and implied odds will almost certainly make it a profitable call, but sometimes I might consider raising as a bluff, because sometimes they will fold, and if they don't you have a lot of outs to win anyway most of the time. It's very situational and it does not come down to direct maths.
Hope that helps,