[M.B.E.]

Thought about this some more. Given that the prize structure is 50%-30%-20%, I now realize that when you're down to the last three, it's usually worth taking a considerable risk of busting out if that will improve your chance of winning. Notionally since you're guaranteed third place money, you can put the 20% in your pocket; now you can see that first place pays three times as much as second. So you would rarely use a strategy designed to "squeak into second place". You want to shoot for first, which means playing pretty loose.

Algebraically, you want to call whenever

p > (3c + d) / (3a + b)

where:
p is the probability of winning the hand if you call
a is, if you call, your probability of finishing first if you win the hand minus your probability of finishing first if you lose it
b is, if you call, your probability of finishing second if you win the hand minus your probability of finishing second if you lose it
c is your probability of finishing first if you fold, minus your probability of finishing first if you call and lose the hand
d is your probability of finishing second if you fold, minus your probability of finishing second if you call and lose the hand

Notice that a and c are both always positive, while b and d could potentially be negative. (b and d might be negative if you have a big chip lead.)

If your opponent has you covered, then a, b, c, and d can be described more simply: in that case, a and b are your probabilities of finishing first and second, respectively, if you call and win the hand; while c and d are your probabilities of finishing first and second, respectively, if you fold. Then you can see that 3c+d is your equity in the tourney if you fold while 3a+b is your equity in the tourney if you call and win (ignoring 3rd place money which is already in your pocket).