Correctly calculate pot odds

[paulw]

Hi all, I always thought I was calculating my pot odds correctly but am now doubting myself. Can anyone advise the correct way?
Example:
Flop comes and I have a flush draw. 33% chance to hit on turn or river or 2:1.
Lets say pot is $40 and villan bets $20 making the pot $60 and $20 to call.
Now, which is the correct method in analysing if I am getting the right odds?
a) I have to bet $20 which is 33% of the current pot, making it neutral EV, or
b) The odds being offered are 3:1, making it 25% of the pot

I think I have just confused myself here over thinking things so if anyone could clarify that would be great.
Thanks!