I've been trying to make myself a rule of thumb for calling bets with gutshot draw.

Let's say I have gutshot draw on a rainbow flop and I think I will have the best hand if I hit my draw. I'm in position against one opp. I want to know if it's +EV to call a flop bet. I need to think about 3 different variables. X = the probability the opp will stack off if I hit my straight. Y = opp's effective stack size. B = the size of the bet.

The equation would be $EV/hand = X(0,08 * Y) - B

Let's assume tight players will stack of 20% of the time, semi-loose 30% of the time and loose 40% of the time. And let's assume the stack size is 40 x bet.

EXAMPLES

1. Opp is tight:

X = 0,2 (stack off %)
Y = 20 (stack)
B = 0,5 (bet)

$EV = 0,2(0,08 * 20) - 0,5 = $-0,18

2. Opp is semi-loose:
$EV = 0,3(0,08 * 20) -0,5 = $-0,02

3. Opp is loose:
$EV = 0,4(0,08 * 20) -0,5 = $0,14

So it seems like 40 x bet stack size isn't sufficient for me to call if the values of X are these. I hope someone has made any sense of this gibberish.

QUESTIONS

What values of X would you assign to tight, sl and loose players?
How would you alter the equation?
Do you think my table image is affecting the value of X much?
Do you feel that thinking like this is pointless, and I should just play by the feel of the situation?
How much do you think the stack size in comparison to the bet should be for the call to be +EV?
Am I making a mistake when I'm not considering pot odds at all but only implied odds?
Do you think all of this applies to the similar situation on turn too?