to call or not to call

[pencho]

Hi there. I have played poker for about a year now and I frequently stumble over the same situation where I do not know if it is correct to call or fold. Perhaps you can help me.
Let us assume the following situation: In a split-limit game I am on the button and limp in, the Small Blind calls and the big blind checks. On the flop, I get a flush-draw with 9 outs (4:1 odds).
Situation A: The first player bets, the second player calls; the pot contains 5 small bets. With my flush draw, a call has positive EV, so I call. No problem.
Situation B: The first player bets, the second player raises; the pot size is 6 small bets. Since I have to call two small bets, I get 1:3 from the pot and it is obvious that I have to fold.
And now comes the tricky situation C: Let us say, I am in the big blind now. The first player bets (pot size now: 4 small bets), I call because I am exactly on the break-even point. But now, the player behind me raises and the small blind calls. Now, is it correct to call or to fold here?
On one hand I argue, that (like in Situation B) I have to commit alltogether 2 small bets to see the next card. So I pay 2 SB for a pot of 7 SB; this gives me a slightly negative EV. So I should fold. BUT on the other hand, if I regard the first SB as a sunk cost that does not belong to me anymore, then I have (like in situation A) to pay one SB for a pot now containing 8 SB, wich gives me good positive EV.
This is disturbing: If I had to pay 2 small bets from the beginning, I would be in negative EV, but now, after first commiting 1 SB and then the other one, all of a sudden I have positive EV on that last call??? So, when playing, do I have to count ALL my bets that I committed to see the next card or do I have to regard the first bet as a sunk cost and only count my odds for the second call?
I hope I have made the problem clear to you. Thanks for any answers.
pencho