The equation: SUM(probability*profit) = EV
Assuming there is no tie:
(chance to win)*(gain) - (chance to lose)*(loss) = EV
*note: (chance to win) + (chance to lose) = 1
Let: (chance to win) = x, (chance to lose) = (1 - x), (gain) = G, (loss) = L
The equation for favorable EV is:
x*G - (1 - x)*L > 0
x*G - L + x*L > 0
x*(G + L) - L > 0
x*(G + L) > L
x > L/(G + L)

In this example, L = 2/3*pot, and G = pot + 2/3*pot = 5/3*pot
x > L/(G + L) = (2/3*pot)/(5/3*pot + 2/3*pot)
x > (2/3*pot)/(7/3*pot)
x > 2/7
x > 28.6%